E30 exhaust leak?

I have a 1988 325ic. For a while I have had this problem where if you drive around with a window down you can smell the exhaust. If all
windows are up or if the top is down, you can't smell it.
I've put the car up on ramps and looked for exhaust leaks and checked the mainfold over with a mirror and flashlight, but have found nothing. No exhaust odor in the engine compartment when idling and engine runs fine under all conditions.
I was wondering if there was any common problem with this car, such as a rubber grommet in the body or convertible top seal that fails over time and when a window is down a venturi effect sucks exhaust into the cabin?
Also in researching the archives here I see that a bad cat gives off an odor. I have 118,000 miles on the car and previous owers may have stomped on the gas before it was warmed up, however my odor is just exhaust with no sulphur or rotten egg smell.
Any thoughts?
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