Used PC is quiet at the seller’s place but noisy at home. What should I check?

Start by separating the PC from everything around it. Try a different wall outlet and a known-good power cable, with no old power strip, monitor, speakers, or other accessories connected to the same strip. If the noise remains, that points more toward the PC or its power supply than a cable or shared setup.

Don’t open the power supply. If the sound is clearly coming from it and continues, stop using the PC until it’s checked or replaced. A scraping noise from a PSU is not something I’d try to diagnose by reaching inside.

The power supply fan is one possibility, but don’t assume the house wiring is the cause just because the symptom changes between locations. A loose part or fan can make noise intermittently, and the sound can be hard to locate. Check whether the noise changes with the case position, load, or startup, without opening the PSU.

If you suspect an outlet or grounding issue, use a suitable outlet tester or ask a qualified electrician to inspect it. Avoid guessing at mains voltage measurements yourself. A PSU that behaves abnormally in one setup still deserves attention even if it runs quietly elsewhere.

That old power strip is an easy variable to eliminate. Test the tower by itself, directly from a wall outlet, then try another outlet in a different room if you can. If it only happens with the strip or with several devices attached, you’ve narrowed down the setup. If it happens on multiple outlets, I’d focus on the PSU and have it checked or replaced.

There’s no clear evidence here that the home’s power is “dirty.” The different result is a clue, not a diagnosis. If the PC is quiet at one place and noisy at another, compare the simplest setup first: same PC, direct wall connection, different cable if available, and no peripherals. If the noise persists or you notice heat, a burning smell, or unstable operation, unplug it and get the power supply replaced or professionally inspected.