I've had the YSM1p for nearly a year now. I as well noticed the rattling. Yorkville told me I should return the speakers because they are faulty but I am certain, now even more so when I saw this that its infact an inherited problem.
It's only an issue with static timbre extended low frequency notes. Basically when dragging a certain range over an extended period of time without it being modified. The cause is the circuit board in the amplifier. This is really not much of a problem since these types of low frequency effects sound very bad in the first place, mainly used in cheap unprofessional trance recordings.
Anyhow these speakers are the best thing I have ever spent my money on as long as I lived. Out of a room full of Genelecs, Mackies, Tannoys, etc these were a clear winner for me, brand new pair.
Don't worry about the rattling, enjoy them. Besides that minor, rarely problematic issue they are pretty much perfect.