"Mike Lenker" > wrote in message
news:wQfwk.581$sq3.384@trnddc07...
> I agree, check all exhaust components, especially heat shields on the
> cats. Most likely due to a harmonic frequency at that RPM. Several
> wavelengths from different drivetrain components may come into synch,
> thereby amplifying themselves through loose components. If there is
> nothing loose on the outside, take a rubber mallet and it each cat (not
> too hard) and listen for rattles. I've had baffles break loose inside and
> cause rattling noises. If that's the case, get a whole new H-pipe and do
> a DIY install. Might be cheaper than a shop cutting out and replacing one
> bad cat. If a shop just replaces one, you'll still have 3 old cats which
> may do the same thing eventually. As a bonus you can get legal hi-flow
> cats and gain a few HP.
>
> mkl
> '86 Capri 5.0 5-spd
> 175,000 mi

Heat shields. That sounds right. And an easy and very cheap fix, too - rip
it out.

dwight