I have 2 engines that have a similar issue and trying get some ideas on what to do.
First is a PS2 MTH engine that still has noise coming out of the speaker when I turn the Engine sounds off. Almost sounds like white noise or a very low level engine sound. Probably closer to white noise.
Second is a Legacy diesel engine. When I turn the engine sound completely off, it seems like the engine sound is still coming out but very low volume. This definitely sounds like the engine when stopped on track. When trains are running, the track noise drowns both the engines out.
All other engines seem to work OK. I think there is something like 14 others and they work properly.
I suspect that either a wire is pinched or possibly shorted. I guess also it could be a loose connector too that is causing a grounding issue. Any suggestions on what to check? Both of these can be aggravating when I am running trains.