hello guys , im new to the forum. i just bought a 2003 altima about a month ago, ive fixed all the major problems ive found, but im still baffled on a loud rattling noise coming from the front of the engine. the noise comes and goes on idle, sometimes you can hear it while driving but its rare. ive changed the oil about 4 times this past month flushing out the gunk from the motor since it seems the previous owners didnt take care of it. i replaced the precat with a header so no heat shield. do you all have any idea what the rattling noise could be?

On modern engines the mounts can do that easily and often the engine mount on passenger side. If you can get engine to do it while you are under the hood, watch the tensioner/belt action while the noise happens, something in the belt stream could be loose. If a 4 cylinder, the normal tendency to vibrate at or just off idle can make the tensioners go into a synchronized shake back and forth as the max harmonic of that normal engine shake occurs, the crankshaft emphasizes that on the belt in a give and take and the belt then rocks the tensioner arm back and forth quickly to make noise and even more so if the belt has worn enough to let the tensioner hit end limit as the action happens. Then it becomes a mechanical striking issue and can break the tensioner really quick. Normal engine shake may be slightly striking the header against something too, new part and until you check out all RUNNING clearances could be that even with clearance at engine off. The engine tends to wind up on the mounts a bit at idle to put it in a slightly different position.

I have a rattle noise that was caused by the power steering resivor rattleing against the engine mount. I do have another rattle that comes when I get the engine hot on the highway. It goes away after I get in town and slow down . The only thing I found it fix that noise is running 20w50 synthetic blend. I’ve tried everything and this was the solution. It’s quiet now and stays quiet. 187k on my engine.