Hello everyone,
First thank you all for the great information I have picked up here in the past couple of months. I inherited my fathers 1974 corvette after he passed away this year and not being much of a mechanic I have gained some courage in making small repairs by reading this forum.

I have a 'rattle' that I cannot locate that is driving me crazy. It happens when accelerating with torque (forgive me if I'm using the term incorrectly). What I mean is if the road is flat, I will usually not hear it happen, but if I am going uphill it happens most every time. The tach is still not working so I do not know the exact RPM but from a dead stop up a hill at about 30 MPH it begins. If i back off the gas, it stops.

Really loud and nasty sounding. I initially thought if was maybe a cross member of the body hitting another piece of stationary metal as it does sound like two pieces of metal rapidly striking each other.

Here is what I have inspected up to this point:

• Inspected all body parts. I did find a nice sized rock tucked in one of the panels. Removed it, but not the problem.
  
• Inspected the engine for any loose items in there and found no problems or items.
  
• Inspected the exhaust and did find a looses bolt from the headers, tightened it but still not the problem.
  
• Looked at all metal fuel lines to make sure they were secure.
  
• Thought maybe it was just bad engine knocking (not that the sound seemed like that but just ruling things out). The car does run nicer after a decarbonizing of the engine but not the problem.

So after another thorough drive, I felt the sound was near the front of the engine and I inspected the cooling fan (mechanical) and it does seem to have some play if I rock it back and forth, not much but enough to notice. Could this be bad bearings in the fan clutch? It appears it is an aftermarket fan. To me, the uneducated it seems to make some sense that if the RPM's are high enough to spin the fan to a point that it looses alignment (due to a bad bearing) this flex-fan could make quite a racquet. and when I release the gas, it slows it's spin and again quiets as it aligns.

Any thoughts on helping diagnose this sound?

Thanks again for all your past advice and information and I look forward to keeping my dad's car on the road with the help of this forum and it's members.

Dave_T

 

Are there marks on the inside of the shroud from the fan hitting it? If you gas and brake at the same time from a dead stop does it do it? Mine did that when I first got it. I once had a duster in my youth that made a racket that turned out to be a couple loose torque converter boots. Good luck.

 

Could be the bearings in the clutch fan or the left motor mount may be bad.
Step hard on the brake and give it some gas and see if the engine lifts and makes the fan hit the shroud.

 

Bluedawg,
Thanks for the quick reply,
I did not notice any unusual marks in the shroud. There is about an inch clearance from the fan to the shroud.
I was able to get the rattle once while standing using the gas and break but it was revved real high almost to the point where the breaks were giving under the power.
This may be something or nothing but when I was able to have it happen while power breaking, I was in my driveway which has a slight incline.

 

Ok, I recant my last statement
After a closer inspection there is one area that does appear to have some rub marks. I also noticed that the shroud is not so much 'attached' and it is 'glued' with an adhesive so perhaps it is not properly centered around the fan.
I'm going to clean up the area and see if it measures the same distance to the fan from all sides.
The rub marks are not what I would call severe but I'm guessing it wouldn't take much contact to create a sizable rattle.

Thank you bluedawg and petes74ttop for the suggestions, I will report back.

 

Dave, if the car is an automatic transmission you can power brake with the hood open. If the motor mount is broken you'll see and most likely hear the engine lift up and the fan will hit the shroud.
Second, the transmission mount could be broken also. You can check that by getting under the car with your hands or a pry bar and lift up on the tailshaft. If it moves its a bad mount

 

Allright I removed the fan shroud extension and adjusted the main shroud to a more centered position around the fan and clamped it down (still need to investigate why it was adhered and not bolted in properly) to ensure no possible clearance problems. Took the car out around the corner to the nearest hill and tested it out.

Rattle is still there and seems to be unchanged with these changes.

PowerBreaked again in the driveway and was unable to recreate at RPM's high enough to begin spinning the tires. Also payed attention to the motor and did not notice (or hear) and indication the motor mount may be bad. I am assuming that I would see a definitive 'jerk' or drop of the engine, or hear it dropping when letting off the gas. So I don't think that's the solution.

I'm going to try and attach the shroud correctly and then look at the transmission mount as mentioned by hugie82. Should I be trying to lift around the differential? What am I looking for here Hugie?

Thanks again for the suggestions.

 

I would also check the exhaust sytem again,(the entire exhaust from front to back) real close......
with the engine off and safely under the car, grab the pipes and see if there is excessive play anywhere in the system. the exhaust runs real close to alot of items under there and when you accelerate, it may be just enoung movement in the wrong direction. and it would also stop making noise when you let off the accelerator. hope this helps......

 

Gonna sound wierd (at least to me), but I had a rattleing sound under accelleration. Turned out to be loose alternator belt. Have a great day, Brgds, Bill

 

Could it be spark knock/detonation? Normally only evident under load.

Friend had a cracked flywheel on his pickup but not sure if it only rattled under load or full-time.

How are the mounting brackets on all of your front engine accessories? Any showing loose?

 

Sounds like spark knock to me. Check the timing and/or run some higher octane gas through it. You will probably find the timing is off,though.

 

flex exhaust connectors make a kind of rattling sound under some loaded conditions.(on my list to replace mine).

 

Check that your damper is not moving and backing into your timing cover!

 

And if that's what it is you better get it fixed before you have engine damage.