If you have been running it to keep it from making noise then you might have a leak on the high pressure side check the high pressure valve. Its notorious to leak. AZ sells a valve kit. My 2001 used the kit #mt2901. Although you could have a leak anywhere could even be the compressor shaft. Dye would be the way to find it for a home mech. If you want you can jump the low pressure switch to see if the compressor comes on but only do it for a second to verify low r134a.