The decoder lists "quiet drive" as one of its features. It also wouldn't explain why it runs silently 3/4 of the time - but not the other 1/4 (and why it would revert to silent after turning track power off/on).
I couldn't find much on the DP5X, so I wonder if it isn't an old/discontinued decoder? If it's as old as the engine (10 years) it may be a decoder made before "silent running" technology came along. Many early decoders had a "whine" when in use. Some have an adjustment CV you can use to make it less noticeable. It does seem with those that it's more noticeable when starting and stopping than when running.
The factory decoder that came with my Bachmann 45-tonner isn't a 'silent runner' and although the engine runs well it does make a similar whine sound to what you describe, especialy at start-up.
I have a P2K engine with a TCS DP5X decoder on a Digitrax system. The decoder will sometimes make an electronic sounding buzz, similar to what a DC engine makes when being run on DCC track. This will start when I'm running the engine. If I turn the track power off and back on, the buzzing will go away for a while. It seems to be totally random when it happens. The engine still runs the same when the noise starts -- it doesn't seem to impact performance, just my sanity. I've had this engine for probably 10 years, and it hasn't made this sound before, though it's had limited run time.
Do you think the decoder is going bad? Or is this an indication of a different problem?
All help appreciated.
Thanks
Todd