I have a 92 Sentra 1.8L engine. When I start up there is a vibrating noise coming from the engine. When I rev up, the vibration noise gets worse. The noise seems to dissapate a bit when the car becomes warmer, but it is still there. Could it be a clogged EGR valve? I am not sure where to start. The car is general is emmitting too much NOx. The catalytic converter was replaced 3 years ago and so was the Oxygen Sensor. I wont pass smog. Any ideas?

My Sentra (2000 SE) was doing close to the same thing.
It ended up being a main bearing in the engine knocking.
I had to drop in another engine.
I took it to a Nissan dealership and the computer diagnosed it right away.
Think it cost me $75 for the diagnostics

My problem is two clogged fuel injectors. Perhaps that is the same problem?