1998 BMW Z3 Slow leak or dying compressor?

Several months ago, I had my 1998 Z3's air conditioning coolant (what we used to call freon!) Recharged. It worked fine for about three months, then winter arrived. Recently I realized the a/c wasn't cooling, so I took it in for a recharge. It's worked fine for about 3 weeks, but sometimes the a/c sounds noisy.

What part of the a/c system would cause this? I had the compressor replaced at around 48K, but here in TX we run a/c a lot.

To complicate the matter, two mechanics I trust have both looked at my car, and one thinks the compressor is going while the other is convinced it's just a slow leak.