2011 Volvo VNL 630 D13 Quiet Ticking that Come and Goes

I'm looking at a used 2011 Volvo VNL 630 with a D13 engine for sale by a private party and as best I can tell it has 650,000 miles.

There are a few relatively minor problems that I can see such as the alignment is bad (pulls gently to the right) and the power steering comes and goes, but my big concern is I heard some very quiet ticking sound while driving.

I couldn't pin it down to any specific speed or engine RPM, the ticking sounds equally loud under heavy load (full throttle at low RPM, or little throttle at high RPM), though the test drive was bobtail only.
The ticking sound would also appear for some random amount of time, then go away for some random amount of time and it seemed inconsistent the length of time it was ticking or not ticking.

It also was certainly a ticking sound like you would expect from a small rock stuck in the tread of a car tire (but it didn't increase in frequency when at higher speed), and not a chatter like if rockers had no oil (though I am not excluding it being a problem with the rockers/cam).
Oil level was within operating range, and according to the dashboard gauges, at operating pressure.

The ticking was very quiet; a quiet conversation or stereo playing would drown it out, and it either never was ticking while at idle when parked, or I couldn't hear it, because I have no clue where it was coming from.

I tried searching, but no one seems to have a ticking sound that comes and goes, so hopefully someone can help.figure out what the problem is.

 

The sound doesn't bother me, what it could mean is what bothers me. If it's an early symptom of the truck needing a major repair almost immediately, I would care very much, but if it's a minor repair or just something it'll do without damage for years to come, I wouldn't care.

I did think of the air compressor, after the drive, so I was never able to check that out, but I did note that the ticking was not happening while parked with the air compressor on or off.
I've never heard another air compressor tick like that, so if it is that, is it likely a failing compressor, or just a unique characteristic of that compressor?

 

@Ridgeline I don't know yet; I've already sent out an oil sample for analysis as part of the normal buying process, and arranged for it to be brought to a dealer for a dyno run and to look it over/diagnose those problems, but was hoping to be able to check out the problems/make suggestions to avoid excessive diagnostic fees.

 

@McMahons Trucking I don't disregard the possibility, but I doubt it.Then ticking would come and go, it wasn't persistent, and it didn't appear to tick at a higher frequency when moving faster.
At 65MPH, I would be surprised if any small rocks weren't flung from the tires as they would be experiencing in excess of 100G acceleration, and the tire would constantly be flexing with each rotation.