Nissan Repair: Air conditioning, receiver dryer, sitting in the sun

Question
I have a 1997 Altima.  The air conditioner makes a noise periodically which lasts a few seconds.  When it is very hot and has been sitting in the sun all day, it is very loud when I initially start out of the parking lot, but gets less noisy as I drive and the a/c cools (I do get cool air).  However, it still periodically makes the noise as I drive the interstate at 65-70 MPH.  It seems to do it when I accelerate.  Someone  told me it could be a dirty blower motor, but I don't believe the noise occurs when the heat is on.  I am not sure if there is a routine to this noise.  I thought it might be making it each time it cycles on?

Answer
Marilynn,

The noise is the compressor and when it is turned on it makes a knocking noise and after it runs for a while the noise goes away and then comes back and goes away.  Since the elimination of R-12 as a refrigerant and its replacement with R-134 compressors do not last as long.  So, if it were me, I would be saving my money to get a compressor and receiver dryer for the airconditioner system.  This will require a system evacuation and refill.  It is your choice when you do this.  If the system fails it will lock up the compressor and that could mean more parts and money.  So, if the compressor is not  lugging the engine down when it makes the noise you have a few more months of operation.  I have seen compressors last for a couple of years making noise or fail in short order.  Now, this is all based on what you have emailed.  So, if the noise is there with the fan on and the airconditioner off you would be looking at a blower motor and not a compressor.  So, check with the fan on and the compressor off.  If the car makes the noise then it is the blower and if it only makes the noise when the compressor is on then you know it is the compressor.