The other respondants are correct in that the "thunk" on start-up is normal. It is not, however, normal once the platter is up to speed. I agree with giving the platter a push as you turn the table on. Although I personally have a P25 which does not have the "thunk" on start-up, my brother uses a P3. We recently installed the new motor upgrade that is now available which gets rid of this noise. While performing the upgrade it became obvious as to why the noise occurs with the original motor. It's mounted, IMHO, in a really cheesy manner with, essentially, rubber bands. The motor literally hangs there and can flop around. I think Rega made a great evolutionary improvement with the upgrade to the P3. Be advised that the upgrade kit my brother got came with no instructions. I would assume that the intent is to have this installed by a competent dealer. Fortunately (for my brother) I have an electronics background and was able to discern where to wire to the new regulator board. The motor upgrade is very solid. You must determine for yourself if it's worth the $150.
Luck to you...Woodrow
Luck to you...Woodrow