Nissan Repair: 98 NISSAN FRONTIER 4 CYL, nissan frontier, compressor clutch

Question
WHEN I TURN MY A/C ON I GET A GRINDING NOISE. IT ONLY GRINDS WHEN IM AT A STOP AND QUITS WHEN I START MOVING AGAIN. ALSO IT DID NOT WORK AS WELL AT THE END OF THE SUMMER AS IT DID AT THE BEGINNING. NOT SURE IF IT MATTERS BUT IT IS AN AFTERMARKET COMPRESSSOR ALSO. THANKS FOR YOUR HELP

Answer
SOunds like your airconditioning compressor clutch is slipping due to your compressor going out.  When the engine is at low rpms the slip happens and when you start moving and the engine speed increases the clutch holds.  If it were me I would be looking at getting a compressor soon.  If you wait and it shells it will plug up the works and end up costing more money.