Hi Keith;
I just had the same thing happen to my 2000 Harrier 3.0 - after a cambelt change, things were a bit rattly and I was really worried!
A couple of weeks later, however, as the weather began warming up (Sod's law!), the aircon wouldn't work - there was a brief drop in engine revs, then the light on the aircon switch started flashing, and no cold air.
I also tried a re-gas, but this made no difference.
Turns out the aircon compressor is siezed; at least it wasn't the VVTI, or something REALLY expensive rattling! I think a replacement is about £350-ish, plus fitting...
However, starting at about £85 including shipping and import duties, there are lots of used aircon compressors available on ebay.com in the USA - depending in the year of your RX, there are several part numbers; my Harrier needs a 447220-3271, which is apparently the same as an early RX300 for the US market, as well as the Toyota Highlander 4x4 and Toyota Camry, which both use the same engine.
I'd like to try a more recent part, from an RX330 perhaps, but as Mine is a Harrier, I need to be careful about any minor differences (there are differences between the Harrier and the RX engine, such as more BHP, but cam belt changes every 62,000 miles).
Eventually, with a little more research, I'm sure I'll find something suitable on ebay.
I hope this helps - best of luck sorting things out!
Roz.