Nissan Repair: Air condition noise, nissan maxima, receiver dryer

Question
After running for about 30 minutes the 1989 Nissan Maxima Air Conditioner gets a low level rattling sound.  When I switch the AC off it stops immediately.  A mechanic said a small amount of lubricant might help and he topped of the Freon with the new compatible stuff.  Should I be worried about this older unit?  It seems to get cold just fine, but when the noise starts, then maybe not quite as cold.  What do you thik it is?

Answer
Jurt,

Run your air until it drops.  The new R-134 is not as efficient as the old R-12 and to get yours serviced with R-12 will cost a small fortune.  That is, think of something like $300 just for coolant and then the parts and labor on top of that.  To get it converted you are out the parts and $15 bucks a can for coolant and there are almost three cans required.  So, if your compressor is knocking save your pennies until it actually goes out.  Now, if it is really howling you can go and get the compressor and receiver dryer replaced and then it is your decision on what refrigerant to use.  Just remember that the new R-134 is no where near as good as what you have in your system.  but, if cost is a factor then go with the R-134 conversion and live with less efficient cooling when at idle.  Once you are moving the cooling is fine.