94 Altima Engine noise and shaking

I have a 1994 Altima...135,000 mi. On and off I have been having bad vibrations coming from the engine...causing the whole car to shake and the muffler is put-putting. The car shakes in idle and the shaking only starts to go away when at full speed, however when I start to slow down the shaking is there again. But this only happens on random days...it hasn't been a constant problem.
Also, just recenlty, I have been hearing what sounds like a chainsaw noise coming from the engine. It sounds as if someone is revving up a chain and then after about 10 seconds it slowly stops. This has only happened while I was idle.
I am clueless as to what this could be. Any help on what might be going on??

Can you hear the noise and get a better idea where it is comming from if you raise the hood? There are timing chain guides on the passenger side end of the engine that make a noise hitting the chain which Nissan says should be removed. Newer KA24DE's do not have them.

If you have a clutch the vibration can be from a bad clutch at the transmission end of the engine.

My engine runs fine and has stock exhaust that I have described in the past as a putt-putt noise.

good luck i cant figure it out either i have a 94 with 151,000 miles and it does the same thing now. i have changed the plugs with ngks the wires with 8.5 accels, a new pcv valve, new egr valve, i changed the tranny filter and flushed it out with new fluid, and i still shakes until you get to speed. i think i need to change my distributor and stuff or maybe my fuel pump because sometimes its hard to start it will start but then die a couple times

I have an automatic ... so the vibration can't be from a bad clutch.
Next time I hear the chain noise I'll pop the hood and see if I can tell where it is coming from. It only happens on random days for whatever reason.
Thanks for your input.

Intake gasket and intake gasket and , oh yeah intake gasket.. They all sound like symptoms of an intake gasket