After replacing a blown header/exhaust manifold gasket and repairing (I hope a small radiator leak), I got the car out today (sorry - it was 65 here this afternoon). Everything went as planned - no exhaust or water leaks, so it is time to hit the road. I noticed some popping when I backed off the throttle on my way out of the neighborhood to the highway, but once I jumped on the highway and backed off the accelerator it was like the was no muffler at all!!! Pop-Bang-Pop-Pop-Bang.... and I'm thinking, WTF??? Did I blow some baffles out of the muffler or what? It sounds great accelerating but when there is back pressure it sounds like a war zone?

Any idea what happened and/or how to correct it without buy new mufflers?