I just bought a 1990 740 Turbo with 109,000 miles. When the car is cold I get this strange rattling sound that seems to be coming from INSIDE the exhaust system. The car runs and drives normally, but when I go under an overpass with the sunroof open I can hear this whoshing sound. Also, sometimes (not always though) when I really nail it I get a loud sound that I assume is the wast gate opening. Is that normal? I'm wondering if maybe the "guts" of the cat/converter are rattling around and sometimes maybe blocking the exhaust? I had a simular rattling problem with an 850 as well as a turbo Subaru, both of which had bad cats. Has anyone had a simular problem? Is the turbo going bad?

Jack the car enough to slide under a bit with engine off. Take a rubber mallet and tap the CC and along the exhaust line. Anything loose inside will usually rattle especially pieces of broken off CC honeycomb guts. If you find broken off junk in the CC, remove it and take a long punch or chisel and hammer and bust up the materal and dump it in the junk. Reinstall and things should be quieter. Of course the car will fail emission testing if required for inspection in your area. It isn't where I live so ran mine sans CC for years until I needed a new exhaust system at which time I installed a new cat. ALso, loose bolts where the main header pipe joins the exhaust manifold will make a rattle that is very hard to trace but seems to be coming from the exhaust line.

bl

I don't know where (what orientation) the cat is in the 740T, but I have seen the "guts" ball up in a stock 240 cat. Made a lump that would intermittently roll back and choke the cat outlet, causing extremely poor running. On stopping, the lump would roll out of the way till next time.
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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67