Seeking mechanical genius to help diagnose intermittent starter problem on 2005 Subaru Forester

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This has proven a difficult problem to diagnose. I have pored over the Factory Service Manual and performed several tests, but because the problem is intermittent, I have not been able to perform tests systematically, and the information that I'm able to provide is incomplete, and there may be some extraneous information in my description here, as well. So I'm hoping there is someone on this forum, a careful thinker with pretty good mechanical knowledge, who can give me a likely diagnosis.

Vehicle

My vehicle is a 2005 Subaru Forester XS AT, ~100,000 miles (problems started happening at around 80,000 or so, I'd guess. The car wasn't very well maintained, but it was usually parked in a garage, it's got some rust, but it was previously owned by a relative with ALS, so she didn't put a lot of miles on.) It's spent pretty much its whole life in northern New England.

The problem

Sometimes, when I turn the ignition to the ON position, my starter buzzes, loudly, and the engine doesn't turn over when I turn it to START. Nothing, just buzzing.
While trying to isolate the source of the buzzing the last time this happened, I took a video for future reference. The video is just the undercarriage, because that's where the buzzing seemed to come from. Later, I opened the hood and listened, and the sound seemed to be coming from the starter motor itself.

NOT the problem (probably)

- This isn't a "click" – it is a loud buzz.
- I haven't had any other catastrophic issues with the car (the wheels probably need to be aligned, and at high speeds, I get some vibration, but I don't think this is related)

When does it happen?

It has happened several times, and seem to be associated with wet weather and parking on slopes, but these circumstances do not reliably produce the symptoms described. It has gone months without issue, and then the problem will occur, and then I will have no issue for some months.

- parked on a slope, front end down (once)
- parked on a slope, back end down, after days of very rainy weather (once, and also my engine misfired a few times, but I was able to drive the car to level ground before the starter problem manifested itself – I think the misfire could be related to the moisture in the fuel system rather than the starter system. I don't know. I haven't had problems with misfiring since)
- parked in on reasonably level ground in dry weather (3-4 times)
- Parked on reasonably level ground in wet weather (1-2 times)

I don't drive it that often.

What seems to resolve the problem?

- Jiggling the connection to the B-terminal seemed to work (once)
- Waiting. I called a tow once, and when they arrived, it started up fine.
- Getting towed (twice I have gotten the vehicle towed, and it worked once the vehicle was set back on the ground)
- The last time I experienced this problem, I was in a parking lot, and since in the past, physically moving the car around seemed to resolve the problem, I guessed that jacking up the car might work, so I jacked it up on the driver side, and turned the key, and the car started.

What tests, etc. have I performed (while everything was working fine)?

- I've misted the wires surrounding the starter, still started fine.
- I checked the voltage between positive and the engine block off, starting, and running, and everything was normal - ~12 off, ~11 starting, and then ~14 once the alternator kicked in. (The battery is about a year and a half old and has been drained once, so it's not in perfect shape, but I don't think that's the root of the problem)
- A year ago, I brought the car to a mechanic, who replaced the negative lead, which he said wasn't delivering quite enough amps. Talked with him and he seemed honest, but the starter problem still occurred later.
- After the engine misfired, I replaced all the spark plugs. I also performed a compression test on every cylinder and there was nothing out of the ordinary, so, as I mentioned above, I think this might have been a separate issue that just happened to occur at around the same time I had issues with the starter (maybe the moisture was the common factor?). Later, I ran a bottle of Gumout through the system.

Hypotheses

- Grounding (haven't looked into this very closely)
- I've looked at the wiring diagram to find all the relays (Haven't tested these, but I looked at them without removing them, and, well, they didn't look messed up in any obvious way. The buzzing seemed to come from the starter itself, anyway)
- poor contact between the plunger assembly and the contacts, but not quite bad enough to just get a "click"
- damaged ignition wire (?) or poor contact between the ignition wire and the S-terminal

There is a diagnostic procedure for Engine Starting Failure in the FSM, but I'm still clumsy with this stuff, and the car is currently starting fine, so I'm not sure I'd find anything anyway.

Possible solutions

- Replace the starter (found them for about $100 on eBay, so not that bad)
- Replace the ignition wire (couldn't find good information about the correct specs for this)
- Replace the relays (about $30 each)

So, I think that's all the information I can give you. I've already done a good deal of investigation, but I'm not a professional, so there could be something else I'm missing. I'd rather not replace things that I don't need to, obviously.
 
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