Engine vibration and flashing check engine light

check and inspect trans mount---exhaust system

Read this post on flashing check engine light

http://ricksfreeautorepairadvice.com/flashing-check-engine-light-is-serious/

There are several other things which can make a noise. The people you showed the car have the advantage of hearing or feeling the noise that we do not have a chance to do.

A catalytic noise can change with the engine RPM. You can rev up the engine in Park or Neutral and should be able to replicate the noise. If not, then you have a moving part making the noise.

Sometimes the Emergency brake material comes loose from the metal backing and jams inside the rear drum/rotors. It will cause vibration when pulling away from a stop, and eventually will grind into powder.

Rear axle differential clutches. There are friction disks inside some types of rear-ends. A special lube additive makes these work. Sometimes a refill of additive and lube can make marginally performing units more quiet.

U-joints can make noise and vibrate. Its another moving part noise.

The fan clutch can also make noise and vibration. The A/C compressor and compressor clutch bearing can make noise. The A/C clutch operates intermittently depending on pressure and temperature.temperature.

sounds like a worn out transmission mount try rocking the ****** up and down and if the play in the rubber mount appears excessive this can cause all of the stated problems

the right outer tierodending needs replaced.

Check for the engine mounting rubber of the side from where the noise is comming.

motor mounts can be changed without pulling the motor the load percent change that the other guy posted is normal and a vaccum leak would cause the engine idle to wander when in neutral. if you need info on how to change mounts let me know

sounds more like a ujoint or a center support for the drive shaft. also check and make sure that the flywheel bolts have not come loose. also check the harmonic balancer and make sure it is not seperating

Was your Pontiac real low on oil, or it just had been a long time since it was changed? I don't think putting off the oil change was the problem, unless you had little or no oil in the engine. Bring it to a parts store to see why the check engine light is on. They can plug a hand held scan tool to the cars diagnostic port under the dash, and 'pull' codes from the cars computer to see why it's running bad. This is a free service at most stores. They can give you advice on what's wrong, and what to do to fix it. Write down the codes, and post them here if you have any further questions. Get this checked asap. Good luck, and let me know what you find.

Vibrations while driving like you are describing can come from tires that have defects (internally broken belts) (wheel weights that have fallen off) or a bent rim (perhaps from another driver [not you :-) ] hitting a curb) Are the tires worn? Have they been balanced and rotated regularly? You report no vibration in the steering wheel.
Vibrations during braking can be warped rotors or discs (but this doesn't sound like your problem since these kind are usually felt in the steering wheel during braking).
If and only if you feel like you can do this safely,
you can try getting the car up to 55, placing the car in neutral, then switching the "engine off" but leave the "key on" (be warned the power steering will go out but the vehicle can still be steered though the steering wheel will be much stiffer) if you try this (do it perhaps in a vacant parking lot or on an empty road) and the vibration goes away with the engine off but while the vehicle is still moving, then the vibration is engine or transmission related. If this is the case you might hope for maybe a bad engine mount. Bad engine mounts can be diagnosed with the vehicle stopped as well. However, if the vibration remains with the van moving forward but with the engine off, the vibration is likely a problem with the tires or suspension. Try to provide some more info.
If you don't feel comfortable trying the above suggestion, please take the vehicle in to an ASE certified suspension and steering professional for further diagnosis.

Finally, I had a similar situation happen to me today. I was riding with a friend in their van and the vehicle started vibrating more and more around 55 mph. We stopped and carefully checked the front tires (including the lug nuts for tightness) but
found a tire defect. (it had several broken steel belts that caused a bulge in the tire and thus the vibration) We changed the tire and the vibration was gone. Good luck to you.