Rattling underneath car. exhust smell

Hi , it sounds like your valve stem oil seals need renewing. If you mix oil with petrol it does not all get ignited hence blue smoke from the oil and the smell of petrol out the exhaust.

Sounds like to me a small exhust leak either
a exhust manafold gasket or exhust flange
gasket or cracked exhust manifold.
Anything else would be enteral meaning
valves need to be set, bad lifter, bad rod bearing,
bad oil pump, or timing chain tensioner,
BUT I would be willing to put my money
on an exhust leak or timing chain tensioner
it's one of the 2

A heat shield is possible but likely a broken Catalyst. It needs to be inspected.

I have a 2003 Nissan Altima making a humming sound towards the front of the motor at idle in Park.

Not sure if alternator or compressor.

Not rattle from timing cover has to be something Nissan already denies.

The rattle that you hear is the upper timing chain tensioners, they can be changed without doing the entire timing chain job, which is really expensive and difficult.

Sounds like gear and chain need to be replaced in transfer case.
The popping and or clicking is the chain trying to climb up over the teeth on the gear in transfer case.

The rattling is a worn-out timing chain (rattling should be louder at one end of the engine). Get it changed now - they will eventually break, closely followed by your bank balance!

Mine does the same thing.
mine will rattle like a chain dragging after I drove it a little while and it heats up. You don't hear it so much when you accelerate as you do when you let off of the throttle I don't hear it so much when I turn right but I do hear it when I go left. I had it in the shop they checked my transfer case refilled it with fluid but I still have the same noise. They claim they don't hear it but they don't drive it Long enough for it to start rattling.

If not differential, ck the rear universal joint on drive shaft by rocking shaft back and fourth. ck for any excessive movement or noise.

Is it a 3.9? Does it sound like it is coming from the front of the engine(timing chain)? From the factory the 3.9 didn't come with a timing chain tensioner. They are available after market.