Right, it could be small parts of rubber/plastic missing off part of the under seat mechanism. I only know this because I've previously had my seats in bits. Would be far easier if I could locate a photo, but since I can't here goes...
The base cushion attaches to seat base towards the rear with a "hook" around a protruding pin on the frame at either side. This has a bit of translucent green plastic around the "hook" to stop metal-to-metal contact. This can fall off sometimes, leading to creaking/groaning from the seat. If you don;t have the green bits, electrical tape around the "hooks" can do the job.
Bit of a faff though as imagine you'll have to remove and partially dismantle the seats.
Also, could be where the springs seat onto the rear of the subframe.