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I have just gone through the whole exhaust system, but can't get rid of a rumble, I guess what Road & Track testers approvingly call "throaty exhaust note." Or what we used to like when we were 17 and sent off to J.C.Whitney for glass-pack mufflers. Now it's not so cool, esp. on a 9-hour road trip. You can hear this drone (definitely engine related) at highway speeds, but also in the gears around town when you push it a little, doesn't take much.
From the top, I took off the exhaust manifold and replaced a broken stud (Brickboard drill and tap method) and new gaskets. There did not appear to be erosion, or even carbon marking to speak of, near the broken stud. I could not see and manifold cracks. Next, the down pipe was replaced by a Volvo dealer 16K ago, according to maintenance records. I don't see any cracks/holes in the downpipe. Next, the cat was replaced with an aftermarket cat 7K ago (records). I also tested the cat (Brickboard pour a bucket of water through method) and it appears to be neither blocked nor hollow. Next, I replaced everything behind the cat with new Starla stuff from Groton. I even tried an exhast tip for a while, on the theory that getting the exhaust an inch behind the trailing edge of the bumper would make a difference - didn't, so took it off. I've been over the joints 3 times and don't feel/see/hear leaks.
What am I missing? Do 740s sound throaty (1987 744) ? My 240 (1985 244) does not (same B230F).
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