1997 BMW Z3 squealing noise

Vehicle began a squealing noise which sounds exactly like a belt problem. The noise doesn't begin when engine cold, but rather after it has warmed up a bit, and increases hotter engine becomes although engine is running at correct temp. Noise will fluctuate lower at idle but does not seem to be dictated by engine speed., Belts all look good and to double check I sprayed the gripping goop on all with no change. Vehicle did have diagnostic which evaluated as cat. Conv. Prob. To trouble shoot cat. Conv. Removed and both o2 sensors with no change in noise. Idles rough but think this is do to leaks in exhaust to muffler.