Pinging noise from bottom front end on start up

[Otto]

Im having trouble locating the source of a high pitched whine/ping coming from the bottom of the engine. It's loudest at idle but when I put it in reverse to back out the sound dimishes a bit with the rpms. I'll put in D and start driving at the sound will get louder again with higher rpms. Also, the engine's lacking quite a bit of power and seems to be excessive back pressure. The sound goes away when its warmed up after a few Kms, but the back pressure persists. I'm thinking about possibly replacing the cat. Any idears?