Ok, I own a 78 Deville that is in pretty good shape, I believe the car is in need for some carb work. Lately when driving and the motor is a under a load I get a loud sucking noise that seems to be comming from the exhaust/ tailpipe area. It sounds like there is a Big vacuum Cleaner sucking away back there. If I go WOT it starts to miss out. At idle and light throttle it runs fine. What gives?

It sounds like a restriction in the exhaust, maybe a bad cat.