Rep Power: 0

I have an '89 Civic hatchback 4-speed, bare-bones base model.

At idle, it's smooth and steady. Couldn't ask for better.
If I put the lights on or turn on the cabin fan, the engine RPM drops a bit, but the idle stays steady and solid at the lower RPM.

BUT, now the whole car is vibrating, the door panels rattle, coins in the glove box rattle, tools in the way back rattle, it's a mess.

The car doesn't have a tachometer, so I can't say exactly what RPM it's idling at. It sounds ok to me though, not crazy low or anything. I think the idle speed is computer-controlled here with no manual adjustment anyhow.

So far my only guess is maybe the engine mounts are bad. Any other hints what might cause this level of vibration in the cabin when the engine is idling steady?

 
Rep Power: 173

Sounds like a pretty reasonable guess. Have you tried looking under the hood or lifting the car to take a look under it yet? How old is your timing belt?

 
Rep Power: 0

No idea how old the timing belt is. Replacing the clutch this week so I'll inspect the mounts while I'm doing that.

Can a worn timing belt cause vibration like this without rough running??

 
Rep Power: 173

Well there could be a lot of causes but an old, worn timing belt with missing teeth and such could potentially be the most problematic!