This just started recently, but on my '05 TL with 145k miles, if I don't use the a/c for a few days and then start it up, I hear a metallic grinding noise when I turn on the a/c. This noise only lasts for maybe a second, and won't occur again if i turn off the a/c and turn it back on again. A/C has been blowing cold, so not sure what's going on. Has this happened to anyone else?

 

the compressor is starting to fail in one way or another is my best guess, or your accessory belt tensioner is on the way out but I doubt it.

The a/c system has refrigerant and oil (PAG ND8 oil) to lubricate the compressor. Over time the oil does loose lubricative properties and moisture can enter the system. Your best bet is to have the compressor replaced and the entire system flushed, and as well replace the condenser and moisture pack. When your mechanic removes the old condenser, they can check the moisture pack and it'll probably have metal shavings in it.