Recently purchased an 89 740 w/ 112,000 miles. Car runs and looks good. Car had exhaust leak at cat. inlet. Fixed that sounds much better at idle. The exhuasr is still way to loud between 45 and 70 sounds like a roaring kind of noise. Most of the time the noise will stop when you let off the gas, but now sometimes it sounds like it is sucking something in when you let off. The rear muffler does not look to be original and has not markings on the underside. Anyone have any ideas???

I had my exhaust manifold and cat replaced, and after this noticed a putting sound from underneath the passenger seat. I discovered that for some reason the female male joint between the cat end pipe and the muffler pipe (muffler pipe sits around cat pipe and is secured via a U bracket) has a slight leak. You can barely see it (more evident when it is cold) and you don't notice it just by looking quickly.

I tightened the clamp down but couldn't stop the leak. Still not sure why this might have happened, but its something to look for since it seems to be common if you have the cat relaced but not the muffler.

-rt

Does the sound come from the engine area or from underneath the car? My '89 740 recently developed a deep roaring sound when I accelerated. It was the flange gasket between the manifold outlet and the exhaust pipe. My 740 has 184,000 miles and it was the second time this had to be replaced.

This is fortuitous, I've had an exhaust that begins to roar about 2500rpm and is definately sounding like an exhaust with "performance mufflers" by 3000rpm.
I cannot feel any leaks, but when I plugged the tailpipe, the car still ran!
Although I still could not feel or hear a leak.
I'm about to replace the system from the cat back, but maybe I should check the gasket described,
How did you find this leak?
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744-16v,745-16v,242Turbo.245DLT

I am having the same exact thing. Sounds like it has a flowmaster on it or something. Plug the pipe and the car runs. I can not find a leak at the manifold outlet but I suspect that is where it is. I already have the gasket for it, since the idiots at autozone tried to tell me it was the donut gasket for the cat. I have checked that spot though and can not find anything. It sounds like it is coming from near the bottom of the engine or under the passenger seat. I am going to find some pentrating spray today and pull the flange loose and check that one. if nothing else I have plans to replace from the manifold back, including the cat. in July. Someone had a cheapo muffler put on and it's starting to rust out. But my noise does start at about 2000 rpm.

Sim,

I am also interested in how you figured out that it was the flange gasket??? Could you tell that was it or did you have to pull it to figure it out?

To make a long story short, I had the exhaust pipe replaced a few years ago at my friendly Midas muffler shop in Alabama. I noticed this noise right after that. After several trips back to the Midas shop I finally took it to the Volvo dealer and he replaced the gasket and the noise went away. I now live in Ohio and this same sound developed a month ago. I took it to the Miads shop here where they checked for leaks and didn't find anything. I then explained my experiences with this gasket and he checked again, more closely this time, and found the small leak there. He replaced it with the proper shaped gasket and the noise went away. I think that it can be a very small leak based on the difficulty the "experts" had in finding it. Cost me $50 to get the gasket replaced at Midas.