I have never had this problem, but im currently stripping some corrado seat frames to fit them to some recaros.
There are 5 springs in total.
1 for the forward adjustment, only a small one so wouldnt think it could make much noise.
2 on the tilt handle mechanisum, again these are small ones.
2 which help the seat recline up and down.
The only two i think could make any anoying noise are the two large ones which actually help the seat tilt up and down, but these are always in tension, even when the seat is in the fully tilted up position. The only way you could test these to see if they are loose is to fully recline the seat and see if the noise comes back.
I may be wrong it could be the smaller springs, but then it might not be the springs at all.
Everything on the frame is welded apart from the runner sections which slide up and down.
Im going on a bit now, but i have a frame sat here in front of me and im trying to figure out where it could be.
Have you had the seats out to see if anything is loose.
if this dosent sort it, i will have another look, and can shove some pic on here if needed.