"dwight" > wrote in message
. ..
> "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

there was a recall on the cats on 5.0 before 92? (free cats)
The ceramic would break inside the cat and heat up, cause funny sounds etc.
Had a 89, one side was broke, would resonate about 3k as a loud muffled
sound, 2.5 to 3.5 or so.
fixed for free by ford.
Also a loose fitting on the exhaust pipe somewhere can do it.