My coupe was garaged for about a week and when started an abrupt rattle was heard/felt for couple of seconds. Any other experience?

Is my engine going bad?

 

check some of the heat sheilds..sometimes they get loose and make a pretty loud rattle banging noise.....

 

where are the shields located?

 

there are alot of them through out the whole under section of the car. It sounds like from your thread that its comming from the engine so they are going to be next to the headers to prevent the heat from the headers from comming inside of the car so look around that area and feel for anything loose.

try to explain to me what the sound is with more detail. it might be something else..the Gs are prone to having alot of rattles