I had a site with persistent video issues and was called in to "fix it." The issue was with Dell Dimension desktops, and I found that when you open the case there is a good-sized horizontal metal bar for reinforcing the card cage along the length of the case, that were routinely being discarded by the local tech staff as "being in the way."
I could not convince Dell to send me replacements for the part, but I can assure you the little bar was important to keep cards from moving slightly in the card cage over time and causing issues such as the orginal poster's. The same Dimension systems, missing the reinforcing bar, would over time have very loud fans. It got to the point that I would just cut a plastic shim and wedge it next to the power supply so that there was less movement in the case if the little reinforcing bar was missing.
I know it is a pain, but after removing the cover on the machine and doing whatever to its guts, if you have parts left over after finishing, re-open the case and put them back where the engineers decided they should be. There is likely a good reason for that part you found "in your way" while adding a new board or whatever. And it is unlikely you can order a replacement after you toss it away.
I finally told the IT head that if his in-house staff kept forgetting and throwing out this reinforcing bar I was going to order a Porsche from the nearest dealer, as thermal drift and the occasional bump and other causes would make this site of 6,000 or so workstations a personal gold mine for me in support charges.
A true tale from the field.