I have an old Bang & Olufsen Beogram RX2 and everything sounds great and everything is working fine except for this anoying thumping sound. I have sort of narrowed down the culprit. It only happens when the platter is on top of the sub platter. Its is not audible unless a record is playing, so it be literally be the smallest possible inperfection. This happens with all records. I think the weight of the platter (it is pretty heavy) is making something slightly touch something else.I know that the sub platter nor the platter are rubbing on anything. Also, if you applt the slightest pressure to the record while its playing the thumping ceases.
I have a B&O TX2 that recently started exhibiting this "thumping," though it really became more pronounced towards the END of every record. Additionally I had noticed some slip or warbling on records. I took the TX2 apart to inspect the tangential arm, and belts, etc. Seemed ok to me (I'm not en expert, just a tinkerer). After reading this thread I thought to look for any physical changes during the end of a record, like a bouncing or something, and GUESS WHAT?
This may or may not be your issue, but my platter is sitting somewhat crooked. So, if you look at your turntable straight on, the record should sit horizontal. On my TX2, the platter seemed to sag to one side (the left). No jokes here please, let's stay on target. Anyway on a TX 2, it's a pretty sleek design and the record actually touches the frame if it's sagging enough. Lo and behold, if the record has just a bit of warp to it (most have a teeny bit), it'll rub every rotation. Thump... thump... thump. It can also create a bit of a warble in pitch, but you have to listen to just the right record to hear this. i.e.: Joshua Trees opening synths make this painfully obvious.
I actually pushed the platter to the right enough times and it stayed level for the most part. I may have to open it up again and fiddle with the little metal clip/shock absorber things to figure out why one has lost is bounce.
Hope this helps some of you experiencing this. It was driving me MAD for several months and I was about to hand the thing over to a tech for 4 weeks. Glad I just looked at it. I thought it would be somewhere on the inside... but it was much simpler.
Now that you have a handle on the problem.....maybe the tech won't need 4 weeks....and may even be able to simply order you the part, if that is what it takes. A little DIY in this case may be OK....