it's my wife's car a 1996 855 2.3T

and it's a groan that resonates under low speed low load braking.

background info - it started shortly after I had tires balanced and rotated. (have since replaced all the tires as they were getting low an tread anyway with winter coming)

There was a rattle in the rear, which the dealer diagnosed and repaired as a loose shock bolt.

after this I'm certaint he groan surfaced.

brought back to dealer and left it for a day and a half, and they couldn't get the groan to reccur and inspected the heck out of the rear end and the brakes, finding nothing to replace.

It was after this that I noticed the noise seemed (but again I don't drive it much) to happen more on slightly rough pavement and figured what the heck, it's the rainy season and the tires are low on tread. So new tires (the dunlop sport a2s) blance and alignment, and the car does feel a little better, but, according to the wife, the noise is still there (she's driving the car now)

is there any other thing this could be? (I asked the dealer to look at suspension parts too, but don't know if they did)