If you're getting Cavitation/NPSH you will be getting airbubbles(small) in the system/tubing, this is what made me think I had it. you might not see them until you stop the pump and tap/knock it, then they rise up the tube, why they 'stick' in the pump making noise and not just flowing straight out I don't know, the noise is a very iritating(high pitch?) scratchy~trickling like noise, it got worse when I had slight pressure on the inlet~housing/tube, if you hav'nt then it's wear or a loose impellor...
If it has'nt
always made this noise and nothings been changed with the system, then you can gaurentee it's not inlet restriction. impellor noise maybe(glue it if it spins), shaft is worn, or a leak are the most probable IMO, strip the pump down and clean it, the O~ring and mateing surface especialy, look for wear on the shaft (if not ceramic) or the impellor housing shaft/magnet assembly that rubs against the shaft, also check the rubber 'grommets' that hold the shaft in place for degradation if additives have been used...
Whatever happens you got to strip it down to have a look see
...