'97 sedan--rattling/vibration from steering column (most prevalent when idling)
 

I have a '97 Volvo 850 sedan that I've owned for about 5 years now, which has been a daily driver for 3 of those 5 years; for 2 of those years I was living in the city and the car wasn't in use.

I've been driving it again on a regular basis, and have noticed within the last eight or so months that there is a rough vibration/rattle that seems to be coming from the steering column. I notice that sometimes when I turn the wheel to certain positions, the noise/vibration will cease, but resume once I move the wheel at all from said point. The car drives normally, but the noise has not gone away.

The noise/vibration is most prevalent when idling, and doesn't seem to occur when the car is in motion. It's especially noticeable at startup or when turning the car off—I've noticed that when I turn the engine off, the rattling noise peters out as the engine shuts off, and the sound almost reminds me of a lawnmower shutting off (a bit of a rough chugging out sound). The noise is very noticeable from inside the car, but is inaudible from outside.

I've checked my power steering fluid on several occasions and it's been full each time. I had an oil change the other day and they said the PS fluid was okay.

Since I've owned the car, I've had the PCV system completely replaced (c. 2015 with OEM parts), and have had to replace the fuel pump relay twice and the fuel pump once. The car has close to 200k miles on it (the odometer has not worked for years, so this is an estimate; it had 135k when I bought it).

As a side note: I've had repeated issues with the check-engine light coming on in this car. In November 2017, a code-reader notified me that the system was running too rich (can't remember what the specific code was). I took the car to a local mechanic, and they did a complete tune-up, new plugs, new sensors (I think the O2 sensor and possibly others?), and this seemed to eliminate the check engine light, but only temporarily, as it's since come back on. The noise/vibration I'm referring to though remained constant throughout, so I'm guessing it is an unrelated problem.

I'm not well-versed when it comes to car mechanics, so I was hoping to get some input from people more knowledgable. Any ideas what this could be?