Hi guys I've got an audi a4 b8 chassis and I keep hearing knocking sounds from the back of my car when going on a high enough gear uphill. For example gear 2 uphill at a reasonable speed. I'm baffled because I've gone to a mechanic but he explains there is nothing wrong. I'm sure it's something to do with the muffler. Maybe the exhaust bushings. Has anyone else had or noticed this problem? P.s. the sound isn't present when going on a flat road, only when going uphill.