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Engine vibration

I have a 95 Geo Metro, automatic, 125K miles, which has suddenly developed a vibration. The car vibrates when hitting around 43-45 mph, at all other speeds it is all very smooth and quiet. I thought it must be in the drive train, but the vibration turns out to be in the motor, as it is present when hitting the required rpm range when the car is not moving. I gently raised the engine at different points slightly to take the weight off the motor mounts, in the hopes of finding one that was defective, but the vibration was still there. I also checked the exhaust system for clearance all around, and that doesn’t seem to be an issue. Any ideas would be really appreciated, thanks!

It could be internal to the engine. I rule out the transmission, because it is present when not moving. Is the engine up to date on all maintenance, including timing belt and spark plugs? How severe is the vibration? Can you live with it? I have a vibration in my '92 Celica’s engine at idle, but, after doing the cheap stuff, I’m not willing to pour serious money into it to look further.

Plugs should be good, but I’ll check them again. The timing belt was supposedly replaced not long ago, but I’ve only owned the car for a few months, so I’ll check it’s condition and tension. The vibration goes through the whole car. It’s not extremely bad, but it’s bad enough that I’m sure it will destroy something in the not too distant future if it’s not addressed. Aside from this vibration it’s a strong running and easy starting motor. Thanks.

Check the harmonic balancer. Sometimes the rubber comes apart.

I’ll check it, this sounds like it could definitely be the problem. Thank you. But more ideas would still be appreciated… and perhaps I should have mentioned in the initial post - the vibration seems to be load dependent when at speed. A little bit of gas, or lifting the gas when it’s vibrating while driving, makes the vibration go away for the most part. That’s why I first thought it must be an axle or wheel bearing. Thanks again.