Sounds like the thermostat is stuck open, at least partially. Going downhill means the engine is under less load and isn’t generating as much heat and because it’s colder weather, dissipates heat more quickly. In warmer weather, the open thermostat allows coolant through the radiator at all times rather than when it’s supposed to at a certain temperature. This very same condition happened to me about 3 weeks ago, just in a different car. Cooled off going downhill and all.
Sounds like you have a parasitic draw on the battery somewhere. Perhaps start by finding the fuse for the alarm and pulling it out. Are there any aftermarket modifications? Dodgy stereo installation? Another possibility could be the ignition cylinder. Often, particulary in older csrs, they can become loose and be in the “on” or “accessory” position when the key is out and you think it’s off. I once had a similar problem with a 1984 BMW 5er. I found a “battery kill switch” that would disconnect the battery when the engine was off. Before I started the car, I would just flip the switch, which was a knob I mounted under the steering column, and it would start right up. Had it for years. Best of luck!
Maybe a test would be to open the hood and run the engine. Have a helper turn the fan off and on a few times, see whether the noise is coming from the engine bay or inside the cabin. Rev the engine some see if the sound changes. If the noise is coming from the engine bay, it may be the AC compressor. Wish I could help more. Hard to diagnose sounds by text! Good luck though.
Does the rumbling “frequency” seem to be affected by vehicle speed? You don’t state how long you’ve had it, but the tires came with the car that sat for 10 years, it’s time for them to go.
If a shop performed an alignment, your suspension is probably in good shape. A lot of (good) alignment shops won’t touch it if the suspension or steering shows very much wear.
“What to expect” completely depends on where you take it. Can you inspect the brakes yourself? Maybe post a pic or two that shows how thick the pads are. A possibility, even if remote, is that a backing plate, which is a cover on the back of each of the brake rotors, may have been struck by some road debris or whatever and is now rubbing against the rotor when the car moves. This all may be something you can sort out yourself for $0 if you’re willing to jack up the car a time or two.
I’ve never heard of spacers or adapters breaking but are the 8 lug wheels a lot heavier than the 5 lug? They were made for a 3/4 ton or 1 ton which usually has stronger axles, diff, and suspension. So you’re increasing unspring weight and stressing those parts a bit more. May be fine though if you don’t tow anything. I used 2" thick spacers once on a 1986 bmw I had, screwed my alignment but I found a local shop that got it right.
Those are absolute peak Mercedes. It’s totally possible to find a replacement fender on ebay or even a Euro-specialist salvage yard in that color.
Welcome to Utah. I moved here from Indiana just over 2 years ago. I really miss breathing clean air. Still have my IN driver’s license, $52 for a UT one isn’t going to happen.
I should clarify that the car had a mystery drain I could never find, and I added the switch to cut off the battery so it wouldn’t lose its charge.