2006 A4 2.0 Quattro 73,000 miles. When driving there is an intermittent knocking noise (like someone hitting underneath the car once with a ball peen hammer) coming from passenger side of car. It sounds like the front floor pan area behind passenger tire, but sometimes sounds like it could be further back. I have eliminated the seat, it makes the noise with or without a passenger. Mechanics have said the exhaust is tight and there does not appear to be a worn suspension component. They could not track down the cause but did hear the noise. The rear shocks were replaced and no difference. The noise happens at lower speeds around town driving and has not happened on the highway. Turns, ramps, bumps, potholes, etc, do not seem to cause it to occur with any more frequency as you might expect, nor does stopping or accelerating. It will even happen driving at slow speeds 20-35 mph going straight. If anything I would say it happens with light turns driving down the street rather than full turns. It is always a single knock then not happen again for a few minutes. It has been happening for several years, but has definitely started to occur with much more regularity in the last month. Any ideas appreciated.