No, the sound is fine - you having problems?
The only problem I have sometimes is that the CD Changer fails when switching CDs and then I get the message CDC Error (really annoying, keep meaning to find out if anyone knows what the problem is). Never thought whether the two could be related before, but certainly the thump does comes from a similar location.
In fact, I had assumed I'd just got used to the thump and didn't take any notice of it before, but maybe it has gone away...if I fire her up tonight I'll let you know.