car has 112k miles has brand new LT1 engine has new dynomax exhaust and new Cadillac converter in and out piece. Tranny flush done as well lots of work done recently. But it rattles in the dash like where the clock is i think that is just loose plastic there but it also rattles on the passenger side down to the left of the floor on the cloth part leading up to the cigarette lighter area/level. It sometimes doesn't though always done in reverse i think and at lower speeds i hear it as well. Mechanics have been under the car while running and said nothing ? i had pulley and tension-er replaced as well starter and battery and all brakes done.

any idea what this could be?