Potential cause(s) for rattle noise as my 340 approaches 3k rpm? *FIXED!*

[blown motor]

@Dakota mine has four blades at 1 3/4" and one blade at 1 5/8".

[benguin]

For the sake of science, I went out and measured my 1970 340 no a/c, 26" radiator with shroud and all were between 2 3/8" and a couple at almost 2 1/2", so pretty consistent- but way more than what you two have.  Very little of my fan blades are in the shroud.
  Ben

[Dakota]

Thanks guys for taking the time to do some measurements.   Interesting... you're only seeing about 1/8" variation in the blade position relative to the shroud.   I've got about 1/2" difference for the odd blade.   I asked Jon from Roseville to take a look at a new fan to see if there's anything different among the blades - hopefully there will be a clearer picture shortly.

[Dakota]

I called Roseville this morning to follow up on my question about the fan.  Ryan pulled a new one from their inventory to look at it closely.   He indicated that there wasn't much more than a 1/32" difference between the position of the different fan blades, so it looks like my fan has an issue.

There was maybe 2 hours between when I called this morning and when I got a call back with an answer.   A vendor that provides customer service like this is a rare and wonderful thing.   We are very fortunate to have folks like these around to help.

I now have a new fan and fan clutch on order.   I'm planning to swap the new fan in first and run it with the old fan clutch to see if the noise goes away.   

[Dakota]

It was the fan clutch.

A new fan clutch and fan blade showed up from Roseville today.   Put on the new blade with the old fan clutch: the noise was still there.  Put on the new fan clutch with the new fan blade:  no noise!   

I think the "tell" on this failure is a rattling noise that starts around 2500 rpm when the engine is cold and around 2800-3000 rpm when the engine is warm.

I don't know if the bent fan blade caused the problem with the fan clutch or not, but I left the new blade on just to be sure. There was about a 20 degree pitch difference between the "bent" blade (2nd picture) and the others (1st and 3rd pic). 

I have to say, driving the car above 3k rpm for the first time is a whole different driving and sound experience.   What a blast!!!!   

[blown motor]

Ok, so you've conquered 3000 rpm. It's time to step it up. 

[Brads70]

Great you got it all sorted out! This is a new one for me, always good to learn new things! 

[dodj]