Bad Noise.......please advise.

[nobrakes]

Got up to work this morning and left my 02S outside last night to drive it in. Temp is 45 degrees and clear. No frost on the car. Started right up and ran great 16 miles to work. Temp gauge normal, no CEL on. When I pulled into a parking space and let it idle all of a sudden car starts to hesitate and hear loud noise from the motor/ exhaust area. It sounds like small rocks and occasionally a large marble tumbling around. I turn the car off wait 30 seconds and started the motor again, turns slowly, but starts and keeps making the noises. I turn the car off again wait 30 seconds and it starts fine, no missing or hesitation, revs fine and the CEL is still off, but making the same noises. Turned car off and went into work worried........Its an 02 S with 49k miles. Just changed oil 1k ago and had a LN ceramic IMS put in 4 years ago at 30k miles. Car runs great and the only warning light on is the airbag light. ........Any insight, info out there? I certainly don't want to drive it making that noise and I hope its not the IMS. The shop that put the IMS in is only 6 miles away..........Thanks in advance, Steve