Question PSU fan on my wife's Dell Inspiron 3650 is making a mechanical noise ?

[6StringJazzer]

It's not just the fan running at high speed, it sounds like the either the fan blades are hitting something or bad bearings. It's intermittent. The noise stops immediately if I pound on the case, but eventually starts again. (That doesn't sound like bad bearings to me). I opened the case and cleaned and tightened everything up once but the noise came back. I fear that there is something misaligned and vibrating inside the PSU and I am not experienced enough to crack it open and avoid getting zapped by residual current.

This is an old machine so if the only answer is to replace the PSU I'm just going to replace the whole machine.

I have built my own desktops but this is the wife's PC that we just bought off the shelf at Best Buy or Micro Center, can't even remember where.

 

[Misgar]

Check to see if any wires are touching the fan blades. I get this all the time when I've been careless fixing old machines.

The noise stops immediately if I pound on the case,

A VERY bad idea if the computer contains a (spinning) hard disk. Bashing the case is a good way to cause a "head crash" and irreparable damage to your data.
https://datarecovery.com/rd/what-is-a-hard-drive-head-crash/

I fear that there is something misaligned and vibrating inside the PSU and I am not experienced enough to crack it open and avoid getting zapped by residual current.

It's residual voltage (on the bulk capacitor) in the PSU which is potentially (pun intended) dangerous. You will be warned by other forum members, never to open an ATX PSU, for fear of your immortal soul.

If the fan inside the PSU is "rattling" replace the PSU (or buy that new PC). If a case fan is making a noise, it's a relatively simple job to fit a new one for less than $10.

If you've never opened the case, there might be a modicum of dust on the CPU fan blades.

 

[BFG-9000]

El cheapo aftermarket replacements for that PSU are in the $30 range.

But I expect most people would use an adapter to put a higher quality, full-sized ATX PSU outside the case.

What a weird setup--the case is large enough to accept a full-sized PSU but Dell only cut a hole large enough for a tiny one.