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

[Dakota]

I'm looking for help on where and what to check on my mostly stock 340 that starts to develop a knocking/rattling noise as the engine approaches 3000 rpms.  The noise is the same whether I'm driving or it's in neutral. I have about 1600 miles on the engine since being rebuilt.  Stock heads, intake, exhaust manifolds.  The exhaust has a heat riser valve which is welded in a open position.   Solid lifters.    Fitech throttle body injection.  4 speed A833 OD bought as a rebuild from Jamie Passion.  MSD 6a spark box and MSD coil.   Timing is 16* initial, with a lockout plate in the distributor for 34* total.  Vac advance is disconnected.  It still has what I believe to be the original harmonic balancer.   New McLeod 3 finger clutch with American Powertrain Hydramax hydraulic throw out bearing.  Running 90 octane "recreational fuel" (no ethanol). 

Oil pressure is about 70 psi cold and drops to about 40 psi once the engine is warmed up. 

AFRs are running 13.5 to 14.1 depending on how hard I've hit the pedal.

Cold and hot starts are fine. 

I'm wondering if there's a wobble in the harmonic balancer that hasn't impacted the timing yet. 

I won't be able to work on the car for a little bit so I can't check anything immediately. At this point, I'm trying to get a list together on what to check when I can get back to it.

[benguin]

My first thought went to the manifold heat control valve on the passenger side manifold, but you've ruled that out.

Does it sound more like a rattle internal or external to the engine?  Possibly from the flex plate on the trans hitting the cover?

[Brads70]

converter melted down? I hope not but I have had that happen.

[Mrbill426]

I was going to suggest converter or flex plate... but then I saw he has a 4-Speed

converter melted down? I hope not but I have had that happen.

[dodj]

Hmmmm. How about a return spring missing on the clutch fork to bell housing so the throw out bearing is not retracted fully?
Pressure plate bolts all tight?
Fan blade touching the shroud? With the engine 'torqued' over a bit a 3k?

[Burdar]

I'd remove the belts so none of the accessories turn.  Then see if it still makes the noise.  You can eliminate the alternator, water pump and PS pump that way.  Does it make the noise when cold and hot?

You can work the trottle from the engine compartment while using a stethoscope to see if you can narrow down where the noise is comming from.  I've heard of people using a screwdriver against your ear if you don't have a stethoscope.  Put the screwdriver against the valve cover and see if it's coming from inside the engine.

[Dakota]

Thanks for all the ideas on potential sources and approaches.  They definitely give me a good list to start.

I have bad hearing in one ear so it's hard for me to tell the direction a sound is coming from, so using the stethoscope will definitely help. 

I'll post again after I dig into this a little further.

[jimynick]

Some more questions- is it transient-come and go- or is it steady and does it continue or get worse as the rpm climb? Is it "not there" at lower rpms?

[Dakota]

I don't hear anything until I get up to about 2900 rpm.  I haven't gone to any higher rpm then that for more than second or two, so I can't tell you if gets louder.   

[Cuda_mark]

I would check the torque on the crank/balancer bolt.


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[Dakota]

   I think I found the problem:    It's the fan clutch.

With the engine running, I used a stethoscope on the valve covers, water pump and alternator and didn't hear anything unusual from those locations when the engine was revved up.

For the next step, I followed the advice from @Burdar and isolated the accessory drives on the front of the engine.

For my 340, there are 2 belts that loop around the crank pulley, a/c pulley, and alternator pulley.   There's a separate belt that loops around the crank pulley, power steering pump and water pump/fan pulley.   I warmed up the engine with everything in place (noise was still there), then pulled off the fan, fan clutch and water pump pulley.   Ran the engine briefly - no noise!   This eliminated the alternator, a/c pulley and probably the harmonic balancer as potential causes.

Put the water pump pulley back on along with the belt and ran the motor again.   No noise!  This eliminated the power steering pump and the water pump.   

The only remaining items were the fan blades and the fan clutch.   All the blades are tight and in the right position, so this leaves me with the clutch as the mostly likely source of the noise.   

If I understand its operation correctly, the clutch is supposed to start "free wheeling" once the engine gets into the 2500-3000 rpm range, which is right where the noise shows up.   The clutch is one of the few moving parts that I didn't replace during the restoration.   I'll know for sure if it's the problem once I get a new clutch from our friends at Roseville.   The good news is that I can put the old one back in and still gently drive the car while I'm waiting for the delivery.

As always, I appreciate the guidance from the members here.   I'll add another post to this thread once the new clutch is in to close things out or beg for more help.

   

[blown motor]

@Dakota I have a new clutch I'll make you a deal on, if it's the right one. Just need to work out logistics. I know someone who's been crossing the border lately and shows up at my place. 

[7212Mopar]

I just tighten my power steering pump pulley last weekend. It worked itself loose and was making noise.

[benguin]

Glad you found it!  Methodical troubleshooting pays off.  I don't think I would have thought the fan clutch could make a rattling noise.   Maybe lost enough of the viscous fluid for some metal to metal? 

Thank you for posting your finding!
  Ben

[Dakota]

It feels like I should have re-titled this thread "but wait! there's more!.

While I've been messing around with the fan clutch, I thought I'd look into finding a spacer so that the fan was closer to the "half in, half out" position on the fan shroud to help with cooling once I have my a/c running.   While taking measurements for where the blade is positioned in the shroud, I came across what appears to be a bent blade.   Most of the blades have 1-5/8" exposed; one blade only has 1-1/8".  Is this normal?   I'm starting to wonder if this difference is causing a balance issue at higher RPMs which leads to the noise I've been hearing.

I have asked Roseville to hold my order on the fan clutch until I hear back from you wizards.