On my 2003 altima, when i crank it up after it hasnt been crunk for maybe 2 hours or more, it makes this loud noise. I can turn my car off, and crank it right bak up and it sounds normal. But it makes the loudest noise after it hasnt been crank for a while. For example, i get up the next day, go to my car, crank it, and its the loudest noise. I can turn it off, come back maybe 10-20 minutes later to go somewhere else and crank it, and its fine. What could be wrong?

ANSWERS: Oil Loss, Buzzing Noise, ETC. - Nissan Forum ' Nissan Forums

forums.nicoclub.com/answers-oil-loss-buzzing-noise-etc-t323273.html

Mar 20, 2008 - 3) Yet another buzzing, rattling or whinning noise. If you own a Quest van, Maxima, Altima 3.5 and some Z/G/FX vehicles, and get a loud ... Mysterious no-spark situation with a buzzing transmission
Mar 31, 2013
ABS brake fault light on, buzzing sound
Feb 7, 2011
ECU making loud buzzing noise
Oct 6, 2009
Buzzing noise under the hood towards the back of the engine ...
May 8, 2008
More results from forums.nicoclub.com

Buzzing noise at 2000-3000 RPM - Nissan Forums : Nissan Forum

www.nissanclub.com/...nissan-altima.../128490-buzzing-noise-2000-300...

Oct 28, 2003 - 6 posts - ‎3 authors
The "buzzing noise"; I can tell that I am not alone in having this problem but has someone figured out ... I fit does, that eliminates the brake rotors, tires, wheelbearings, CV's etc. ... BUT with an Altima it is probable the pre-cats. Loud humming noise when driving
7 posts
Aug 29, 2009
Noise when accelerating
13 posts
Jul 21, 2009
engine noise/whine - 05 3.5SE
15 posts
Nov 17, 2006
My altima has a humming noise under my hood
15 posts
Jan 17, 2005
More results from www.nissanclub.com

By chance did you check your oil level. That would be ideal before your engine blows out. If that is fine, more in likely it maybe normal. Some GM vehicles do that also due to the cold weather. After sitting for a long period of time the oil will settle in the oil pan til you crank up. The knocking noise may last for 5-10 sec then it stops because the oil is lubricating the pistons again. But if it is constant and oil level is good, get a mechanic to check it out. Hope this helps.

Wheels, brake binding, prop-shaft universals, half shafts, and loose wheel nuts.

more than likely it is the starter. I would take it in and have it checked, you dont want to be left stranded somewhere with a car and no starter. The gear teeth are probably chipped

you call it the serpentine belt, we call it the fanbelt.it,needs tightening,slightly loosen the alternator, then pull away from engine and tighten belt, tight enough when you can move longest part of the belt, between pulleys, 1/2 inch in/out.
if squeel re occurs fit new belt.

Just take it into the exhaust people and they will rehang it with new surports plus the muffler might have baffles broken .

When you put your A/C on, there is Vacum Motor or Solonoide, that increases RPM of the Engine to compensate the load of an A/C. If the vacum hose is leaking or raptured the RPM of Engine remain low and Engine Shakes. When you put the A/C on and the RPM of Engine Increases, but a little, then your Selonoide is working but not adjusted properly, so adjust it for the proper RPM of the Engine. Put all the loads on, I mean Head Lights and A/C on and then ajust the RPM.

1st check the the oil level and the vehicle's oil type.(owners manuel) 5w30-10w30
10w 30 is a thicker oil and could possibly be causing this.(Moves more freely through the engine heats the oil)

check the drive belt and the pulleys

replace crank-positioning senser