What is that noise at 3000 RPM......

[John Donohue]

Lets try another topic besides electric.....my 77 spider runs well, but in the range of 3000 to 3200 RPM, it makes a noise...hard to describe...kind of like a loose metal pan vibrating against something. I am fairly certian it is coming from the engine compartment. I did a search, but nothing seems to be obviously loose.... Above and below this engine range, nothing.....really weird. Any advice?

[pertyfly]

In most cars, when you have a buzz or rattle at a certain RPM range and not others it's almost always exhaust related in my experience. My first guess would be that the heat shroud on the manifold is either loose or broken. I know when I took mine apart, it had broken around the bolt heads, where you really couldn't tell until you removed the bolts.

Now, also from experience, when you have it vibrating in such a small range (3000-3200), if you leave it this will become a larger and larger range (as it gets looser) and it will start happening at lower and lower RPMS

If not that heat shield, try just looking down the exhaust mostly to see if it's at least close to something. Again, it's almost always a heat shield somewhere

Hope this helps!

[mbouse]

I, too would strongly suspect a heat shield rattle. My '80 and I were never so happy as to remove the catalytic converter which had its own heat shield attached.

We can now rev better, and no annoying rattle to go along with certain torque ranges.

[swensonsj]

Wow! I have the exact same rattle at 2900-3000 RPM. I replaced the exhaust, from the front muffler back, this weekend hoping it would go away. But it didn't. I'd be interested to know if the original poster ever diagnosed the problem. I'll start my search of the heat shields this weekend.

[RoyBatty]

Check the security of your cam belt cover and the alternator on it's brackets.

[Exit98]

Hi, yeah what Roy said first. Tighten down the upper and lower alternator bolts. They need to be tight.

Secondly, check where the emergency brake cable passes under the exhaust pipe in front of the passenger rear wheel. Mine bangs around like crazy. I'm working on a fix.

And please be sure to let the board know if you find a fix. Its frustrating when posters help to diagnose a problem then the thread dies without the original poster ever posting a conclusion, even if its inconclusive.

Good luck.

[katsi]

Exhaust - get rid of the cat if you can.

[DUCeditor]

Mine had a similar buzz since the day I got it. Not so much at a constant speed, but when I revved up through the gears as it passed the sensitive RPM.

At first it didn't bother me because my first experience in a small sports car -- a Triumph TR3 -- had the exact same phenomenon. Just part of the experience thought I.

But in time it came to bother me and search as I might I never found the culprit.

Then late last summer I replaced the rubber 'donut' and lo and behold the sound is almost gone.

Not that I think sound originated there. More likely, I expect, is that the vibration that excited whatever it was that made the noise (it sounded like it was right behind the firewall) is less and thus the sound if it appears at all is at a lower volume and for a shorter period.

-Don

[Akgriffith]

I know this is an old thread, but I too had a rattle from 3200 on up. I tracked it down to a couple loose bolts on the timing belt guard. Gave them a few turns and no more annoying rattle.

[DieselSpider]

That takes care of the symptom but does not always get to the root cause which many times is either a failing alternator or timing belt. Yes check for loose bolts and such but also check for belts that vibrate due to the cord failing inside them and replace them before they come apart.

I have had new belts that had been on the shelf too long start to vibrate a few weeks after installing them even though there was no external signs such as cracking of fraying however after they heated up they became like rubber bands since the internal cords were failing.

[RRoller123]

I need this thread! My '80 FI has a short rev range around 4100 where it does a strange noise that I guess I would call a rattle. Pass through it and it quiets right down. Maybe 200 RPM max. I have though that it was the Cat springs hitting a harmonic, but they are good and tight, as is the entire system. Maybe something internal to the Cat?

Pete

[NSFIAT]

I may be looking at the same issue and will check out some of the points mentioned above. I've definitely done a quick check and made sure that things are tight, but I may be overlooking something.

2nd & 3rd gears (usually), around the 3,000RPM mark - sounds like a bunch of small coins vibrating at a high frequency in the area of the exhaust manifold / air intake. Goes away when I'm above that range or when I shift gears.

[RRoller123]

Mine is actually more of a screech, actually, as much as a rattle. But the fact that it is in such a narrow band indicates a harmonic effect. I need a passenger to hang out over the side and see if it can be located.

[NSFIAT]

Update - heat shield was the issue. Pulled it off and went for a quick drive before things got too hot. Are there any aftermarket options or photos of shields that others have fabricated?

[KevAndAndi]

Glad you found the source. My engine lacks the heat shield, so my source of a similar noise was the rattling sheet metal on the backside of the timing belt cover. Removing all the pieces was very satisfying.