strange sound in my car

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I have a problem in my car mercedes-benz cl500 model 2001. The problem is: When i start the engine i hear a strong sound right under the driver seat and the sound changes with the gear shifting and that sound cant be heared from outside the cabin of the car only when your inside. I took it to the mechanics and he said the soud comes form the gear while checking for that in the gear the mechanics switch off the car and removed the belt then started the car again but this time there was no sound. he suggested the problem with the power steering, I bought a power steering ( used one ) and replaced it the sound ws a little bit lower than before but within hours the sound was back like before, right now with using the old power steering or the new used one that i bought when i try to left the car up useing the hydrulic system of the car's dumpers while it's lefting that sound disappear but once the lefting operation stops that soud is back again. Also when leaving the car un used for couple of days the front right side of the car drops down while the other side is still the same. Note: the sound problem started 9 months ago when i bought the car. Your kind and propmt reply would be highly apprecaited. Thanks Ayman