How do you know if there is a vacuum seal problem?

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. The humming sound could be an exhaust resonance from a heat shield or something but we would need to drive the car and hear the noise or it could be a vacuum issue causing a seal to resonate hard to explain but would need to carry out tests to identify. You would need to go for a drive with a mechanic and point out the noise when it is occurring. The oil light issue would worry me. You should verify the oil pressure by removing the oil pressure switch, screwing in an oil pressure gauge and verifying the oil pressure meets specification at idle and at engine RPM's or you could try changing the engine oil, oil filter and oil pressure switch and see if the light now remains off. Doing this, you will eliminate a failed warning light switch and the possibility of a restricted oil filter.

I think that you have a couple of different problems going on their . That car line has problems with the plastic intake manifold going bad so you could have a vacuum leak. But I think that your problem or problems might be a little deeper than that. But to answer your question on how to find a vacuum. The best way I how found is to use a smoke machine but I bet you do not have one. So one thing you could do is to use a fuel that the motor will burn. One thing is carburetor cleaner. Make sure your engines ignition system is in good order. Start the engine and let it idle. Spray the cleaner in the area that you suspect might be the problem. If the engine changes rpm in anyway you most likely have a leak. Make sure that you have a fire extinguisher ready if any problem was to arise.