First of all, I have pretty good hearing for an old guy, always have had. If you don't have a TMS to track your tire pressures, you really don't need to follow this story further.
Planning for an upcoming fishing trip to Florida in Febr., I swapped out garage stalls between my bass boat and the SSR. The SSR had been in my #1 garage stall which is climate controlled and insulated and I'm out there quite often with the overhead lights on. The SSR got moved to garage stall #3 and both #2 & #3 are not insulated and are usually dark. Made that move about 6 days ago.
Took the R to the golf course this morning and while sitting sideways in the passenger seat to put on my golf cleats, I heard an audible (whine) and could not figure where it was coming from. The R was shut down and nothing was "on". Could not decide if the (noise) was really in the truck cab or something close by outside.
Played 18 holes and returned to parking lot where we all have to drive across to another clubhouse for our (19th hole summary of the round and beers). Got in the truck and heard that whine again. Made sure the radio was off and all else was off and made my way to the other site. Once there, I was determined to track down the source. Yanked the Escort Radar Detector off the windshield and decided that was it. Wasn't sure why, but went on inside.
Came back out and got in............Whine still there. Only thing left on dash board (directly beneath where my radar detector sits) was my Tire Monitor deal which is mostly (Solar Powered). The screen on it was totally black meaning the battery was dead (or as it turns out........not quite).
That damn "hum"/"whine" was coming from that tire monitor unit.
I dug out the charge cord that came with it and now have it re-charging and while still in the (dark) garage stall will likely leave it plugged in.
Anyway, problem solved, your own results could vary.