The sound is more like a pop/thud from the speakers when turning on the ignition - not every time but it's extremely random. Have had the RDX back to the dealership four times to try and solve the problem. First, they couldn't replicate the problem, so they replaced the L/Air Mixer but problem not solved. Then they replaced the Mode Control Motor. Still not solved. Left the vehicle for two days with dealer so they could hear the noise and they decided that audio system needed to be replaced. That was done and the sound was more amazing than the first drive home ......but the popping noise has occurred twice since the system was replaced. We have been sure to shut all electrical items off at the end of each use/trip but the problem still occurs randomlywhen we start the RDX. We have been using the Nav system and are currently suspecting that the problem may be linked to that. When using the Nav system and turning car off there is no way to shut off the Nav system. You can end the trip, etc. but the map will still be on the screen when the vehicle is shut off. Our thinking is that audio is still on for the voice from the Nav system. I asked the dealer if the Nav system can be shut off, like the radio, when shutting off the car and was told no. So we are puzzled that we not been able to solve the problem.
Otherwise love the RDX - have had some great trips through the Rockies.