Wikipedia, source of more questions...

Classic (model years 2001-2003) have a steel body.
2G (model years 2004-2009) have an aluminum hatch and hood.

Classic and 2G engines have serpentine drive belts. The Classic belt turns the AC compressor. If the gasoline engine is not running then the compressor cannot be spinning.

If the camshaft sensor was bad, the engine would not run. The Prius engine sounds much like a sewing machine. If your engine has logged many miles, perhaps the valve adjustment is loose. That can be fixed but is expensive because the camshafts have to be removed and the cam buckets (which provide the correct clearance) replaced where the spacing is out of spec.

 

No aluminum that I know of and the steel doesn't seem remarkable. However, CNW Marketing tried to claim the Prius was made out of some sort of 'unobtainum' steel. Only the original Honda Insight had a body or body parts made out of aluminum. Try a kitchen magnet.

There is a very thin belt that connects the crank pully to the water pump and A/C compressor which has an electrical clutch. The A/C runs only when enabled or if 'defrost' is enabled.

Patrick had made some efforts in adjusting the valves in the past. Also, I know some folks have installed sound deadening material to reduce the noise. However, I seldom run the ICE over 3,200-3,300 rpm so I seldom notice it. The reason is I prefer to keep the ICE in fuel efficient mode:

Anything over 3,300 rpm starts to get inefficient and above 4,200 rpm, downright wasteful.

Bob Wilson

 

Parts of the body (and possibly chassis) on all Prius models are of ultra-high-strength steel, which was used for weight reduction.

I suppose that engine clatter could also be due to the (metal chain) valve drive having worn, or its tensioner no longer 'tensioning'. However, neither this nor the cam bucket wear that Patrick mentioned are at all common in Prius. I would like to make sure that the engine oil pump is working well, given the symptom of engine noise.

The camshaft dedicated to intake valves does an angle-change trick in your Prius. Certainly if it was way off it could prevent engine op. But maybe it is only a little off?

For 2001 and 2002 there is a SSC for free replacement of the crankshaft position sensor. Has this been done on your 2002?

When operating under heavy load, I'd say that the Prius engine makes a bit more noise than sweing machine Some of us added sound-absorbing material under the hood so we wouldn't have to hear all of that.