HELP!! 1992 300E 4matic oil gauge drop and rumble

Thank-you in advance for your respond. I have a 1992 300E 4matic with 125,000 miles. The car runs great except when I stop. My oil gauge reads 3 at start of engine and traveling, however, when I stop, the oil gauge would start to drop, once the oil gauge reading goes below 2, the car starts rumbleing so loud that the car next to me can hear it. The gauge has not dropped below 1 1/2. The sound comes from the bottem rear seats. At first, I thought it may be the breaks until I start noticeing the symtoms occurs only when the oil gauge starts dropping. The oil gauge would go back up to 3 and the rumble would stop once the car starts moving again. Please help!!! Thank-you thank-you thank-you.