The kind in the 90s with a giant CRT monitors and noisy hard disks & fans. I think "buzz" and "hum" are not loud enough because nowadays laptops still hum when overheated. But I don't know a stronger word. Thanks for any help!
You might consider whir, which Collins defines as:
whir (or whirr) (n.) a prolonged soft swish or buzz, as of a motor working
Another possibility is drone. The word drone has many meanings, but it can be used as a noun or a verb to describe the noises you mention. From Wordnik:
drone (n.) a continuous low humming or buzzing sound
drone (v.) to produce a low-pitched hum or buzz
As a side note, I'm not sure I agree that buzz is "not loud enough," because buzzing covers a wide spectrum of noises, some of them rather loud:
Though we spotted tracks, we saw very little wildlife; the buzz of the machine tended to scare animals away.
Source: Snowmobiling Mont Tremblant, Quebec, by Ben Abramson, special to The Washington Post, Sunday, January 28, 2001
As mentioned, the words: whirr, drone, buzz, hum describe the nature of the sound - but don't say much about its volume. You could opt for a word that is less descriptive but implies loudness like "clatter", or you could use hyperbole (which is a tendency of many US native speakers) as in: "That old computer sounds like a helicopter."