My 1977 Jimmy with 400 small block had a kluged together dual exhaust system with pipes ranging in size from 1 ½” to 2 ¼” with 2 ½” in/out mufflers. The mufflers were an off brand called “Mean Streets” which actually sounded “good” with this exhaust system, pretty quiet at cruse and a raspy growl at WOT. I rebuilt the engine and installed street headers and a 2 ½” dual exhaust system with no cats. The exhaust exits at the stock locations with the tips turned down. The sound went from “good” to outrageously loud with so much resonance the door panels literally rattled and vibrated at 50 MPH. I decided to switch to SpinTech mufflers as I had ridden in cars using them and really like their sound. To be on the safe side I bought their RV mufflers as they are the most quiet and are advertised “to give you the performance you want, without sacrificing interior comfort you enjoy. Has a great sound under acceleration, yet at highway speeds you hardly hear it. The resonance is somewhat reduced but the exhaust noise is still crazy loud inside. Twenty minutes on the freeway and I want to jump out and walk. The sound literally hurts my ears. Forget talking with anyone or turning on the radio. Having had good experience with Spintechs in cars this was an expensive disappointment. I’ve done a bunch of forum searches and found mention of the K5s being loud though the floor. So what to do? Switch to Hemi style Turbo mufflers like Dynomax makes. Install Dynamat (expensive). Spray on the “Quiet Car” goop (also expensive). Does anyone have experience using Quiet Car?