If you buy used and they are out of warranty and you consider yourself as a skilled tinkerer, there is a thread on RC that goes over replacing the bearings on them to quiet them down. Buy some really good high quality RC bearings from Boca. You'll need a couple tools and a bearing puller to do it without damaging anything.
I have the bearings downstairs for my 1.5 yr old MP40, I just need a bearing puller and then I can do the replacement.
If you do decide to do this, just don't use a screwdriver to pry off the bearings as it will for sure break the magnet.
No guarantees on how much quieter this will make them though.
P.S. In theory you shouldn't need to do this as the product should be engineered to be quiet. But as we all know, they slightly lack in this area...