2000 Camry--trouble starting, then clatter under hood after keys out of ignition

Twice, (a month apart,) my car wouldn’t start. The first time was mid-day after I had driven the car about 8 miles, parked it, and then went on an errand for about an hour. After trying the key a few times without success, the car started up and everything was okay. This morning, (Sept. 2, 2009–8:15 a.m.) the car was in the garage. I turned the key in the ignition and nothing happened–there was no grinding sound–but dash lights turned on. Because I thought maybe the car had actually started, (I don’t hear that well,) I put the car (transmission???) in reverse and the car didn’t move. I tried to start the car again (in park,) and this time the car started to shake. I took the keys out of the ignition and got out of the car. The car was rattling (shaking) and it sounded like a propeller was hitting the inside of the hood. Then the car was quiet and still. I put the key in the ignition, turned it on, put the car in reverse, and drove out of the garage. Our long-time mechanic can’t diagnose the problem unless it happens when the car is with him. (My husband said the mechanic told him to drive MY car for a while and see what happens–I think my husband just likes my 6 cylinder 2000 Camry better than his 4 cylinder 1999 Camry!)