I had major squeeks with my maggies on my 36. It happened with any rim. I semi-solved it by hot glue gunning sandpaper to the pads; it still let the uni stop, and ground away the powder coating.
From Maggie FAQ:
Q - You guys keep bragging that Magura brakes don’t squeal. I just fitted new ones and they squeal a bit. Why are you guys lying?
A. Noooooo, we’re not lying. Well, maybe a little. My HS-33’s squealed somewhat when brand new until the pads wore in a bit. I’d suggest taking sandpaper to them - put the sand paper on a flat surface and rub the pad on it until the factory finish has gone. Also scuff the rim walls with 3M Scotchbrite pads too. Mine have never squealed since. What Maguras don’t need is toe-in like V-brakes. Set the pads dead flush to the rims. Remove your tires to really see clearly what you’re doing.
Big Dave’s additional comment - Another thing that may cause squeal is lack of boosters, air in the lines (not getting full power) and loose bolts.
Hope you find a solution; it’s likely my KH29 will have the same problem.
Yea that sounds about right, mine are now worn in and like perfectly flush and mine are silent, rode with them yesterday, not a sound.
I decided to grab the pig by the b****, so no half measures this time.
I’ve:
- Sanded the rim (it had a very rough surface with tracks in it and a nasty weld join which caused thumps every revolution). If i where to guess the 08 rim is to blame (the 07 version is nice and smooth)
- Sanded the pads (most likely not the culprit, but got rid of rims imprinted “tracks”)
- Rebuit the whole wheel (stock build was off center and out of true)
I’ll:
- Get a brake booster
- Adjust the pads more precisely
And if that doesn’t solve it I’ll just live with it - hopefully I’ll get my booster this week so I can give it a try. (I won’t check what did what but report on the final result.)
I had the same problem!! And I know the feeling of people turning around thinking it’s a firetruck with a major emergency!!
I used normal chain oil to lube it but it only last for about 60km then it squals again…
Hey,
My brakes usually squeal when aluminum oxide from the brake surface has gotten into the pads. For me, cleaning pads and the rim with a degreaser like Simple Green, and perhaps lightly sanding the pads, does the trick.
Kris
After saying how quiet mine were, they have now started to squeal realy badly. I think it was after they got damp in the rain yesterday. They have goten wet many times before and stayed quit though, weird.
I think I may try that, although everyone stares anyway, why not tell them you have brakes by making a little noise?
Pleeeeeease dear mailman give me my booster today so I can try it out during the weekend. 
Let us know what difference it makes. I will be suprissed if it makes much difference, but it may. The KH29 frame is amazingly stiff, compared to my brothers bike, there is no flex in the frame at the breaks. My brothers bike frame flexes loads, my 07 KH29 doesnt look/feel like it flexes at all.
Although saying that, when I adjusted my breaks so they were only just off the rim, I think I got them to touch as I pulled away.
I put everything together during the friday, but there was no motivation to ride because of constant raining… 
From just goofing around in my apartment it seems that:
- Frame flex almost nonexistent (<1mm, can’t see any) now (before it flexed about 3mm on each side if I squeezed the lever firmly). So whilst the KH is stiff the Maguras put lots of stress on any frame (even 2wheel trials frames use boosters and they are designed for a sport in which you really need a lot of braking power…) You really hear the distintive clunk now when the pads contact the rim and the the lever will not budge any more.
I expect the feel to have improved and the noise to have gotten more dull, as this is what happens on trials bikes. - The horrible squealing has subsided, but I’ll not assume it’s gone until the pads are worn in to then new adjustment…
- Sanding the rim seems to have made the biggest improvement, but since I did all at once I can’t verify this.
I’ll report back once I’ve gone for a proper ride. I’ll try and post a pic also, the booster looks really nice on! 
mine don’t make any noise - they are sitting in a box- I don’t miss them.
Update with pics. Haven’t ridden anything this week due to constant raining, but hopefully will ride a bit during the weekend.
Wow, that looks like a beast of a brake. Do you know how much the booster weights?
52g according to the manufacturer (without bolts). This is one of the lightest boosters on the market (possibly even the lightest stock?). I would say weight is not an issue though, I was mostly curious to test a superlight 2bolt booster, having used what is probably the buffest 4bolt booster on the market on my trials bike way back when…
Anyway it seems to do the job good enough. (I’ll “field test” it properly today.) The main caveat is the price, is cost me more that half of what the hs33’s cost me.
This is probably a better option in reality:
Same high price tag though.
I guess for half the money this one would be my recommendation, it’s thrice the weight though (but that won’t matter) .
To clarify, this is not the first resort when it comes to silencing a loud brake… it choose to add a booster cause I also wanted to elminate to spongy lever feel. However the brake works perfectly fine without, as picky is not a nice work I’ll go so far as to call myself a connoisseur instead. 
I just installed an HS33 on my KH24. No squealing so far, but from the world of biking, playing with toe-out and toe-in and sanding the pads and perhaps the rim are the best solutions. One of those has to work.
I just got my KH24, and got a 2007 when I thought I was ordering a 2008. I was curious about the comment about the better rim on the 07 KH29 and wonder if it also applies to the KH24 rim. As I’m still a bit of an idiot, I apparently have to talk myself into being happy with the 2007 thing. But even if not, yesterday’s ride (my first on it) was very helpful toward solving this (internal) problem!
Haha, I knew I am an idiot so I bought the 08 instead.
The main motivations for the 08 are adjustability in height on the frame. Also the new seatpost is very nice and with the brake adapter the lever comes closer to the handle which is nice.
The tyre is VERY nice also. Other then that the upgrades are very small, like shaving a few grams of the rim. The pedals are also very nice, but seems to have zero durability. They are made of cheese and painted with water based paint it seems.
Pad Orientation to Reduce Squealing
I realize this reply is about 10 years too late, but I recently found this thread useful in trying to solve squealing on my rim-braked G29.
I started with an anodized Surly Rabbit hole rim (a great modern tubeless comaptible rim that is rim brake compatible) and a Triton triple frame with a magura rim brake.
Build Details here:
Squealing was so bad I could feel the vibrations in the seat.
Sanding the anodization off of the rim, and sanding the pads slightly (suggested in this thread) reduced the squealing a lot, but not completely.
After much trial, error, and bitter cursing I stumbled upon a solution to the squealing - pad orientation.
I found that if the pads were perfectly parallel to the rim at center of the pad then the squealing would stop.
In other words, If the pads were angled so that the front was even slightly higher up the rim than the back (or back higher than the front) the pads would squeal.
Hopefully this will help any of you rim brake hold-outs still out there.
If your wheel is true, remove and inspect your brake pads, and replace if significantly worn. Otherwise, if they are slick or have some uneven wear, use a sanding block with 100-120 grit to even/level out, and remove any shine, then remove any residue with rubbing alcohol. Reinstall pads and toe them in slightly, in direction of travel. Clean rim sidewalls with rubbing alcohol. Works for me.
I also found that one of my Magura clamps was a tad loose, which also contributed the vibrations and squealing.
I appreciate the good brake pad advice above. I agree that this is the most obvious cause of squeal. I forgot to mention that I had already cleaned and sanded the rim and pads before taking the other measures in my posts.
I know toe-ing in the pads works well for typical rim brakes, but I’ve heard conflicting advice as to whether it works for the Magura hydro rim brakes. Ill have to experiment with this a little sometime.