Audio noise while car is on no prob in past

Question

I have had my car stereo for about 4 years and had never had a problem with it before. Now when I'm driving down the road my stereo will make a loud thump thru all of the speakers. I have 2 amps attached the newest one is about a year old, but this noise is thru all of the speakers. I checked the connection at the battery and to the amps and they are all fine. What could be causing this loud noise to come across? It makes me and everyone just in the car when it does it and I have no way of knowing when it will do it next. Sometimes it's fine and sometimes I just have to switch it to standby.