LMM Cold Start Thumping/Knocking

[The owl]

Hey guys, I have a weird noise that I wanted to run by you all.

On cold start the last 2 mornings (sitting overnight <5 degrees outside on start up). The trucks starts fantastic then goes into high idle. When I get in the truck I have heard a knocking/thumping sound (consistent with RPM) in the cab towards the center of the dash. I cannot hear this same noise outside or under the truck. As I put my foot on the brake it drops to normal idle and goes away, then won't come back until the next morning. Drives great, no smoke, still gets same MPG (18-19).

Here is specs that might matter on the truck.
- 08 LMM qith 138k
- EFI Live (Idaho Rob Stock Tune, dsp5 switch)
- Intake and exhaust
- PPE Rail Plug
- Cat Filter (<2000 Miles since change)
- Running Rotella T5 - 15/40 (<2000 Miles since change)
- Balance Rates (In Park) on 1-7 are all +-0.2 except Inj 8 is at +2.1 (has been like this since I got my edge cts2/put efi live on at 75k)
- Oil Pressure/Coolant temp and levels are normal

Any ideas? I don't want to sound like I am overreacting but I like my used stuff to run like brand new.

I should add that the exhaust note is consistent/normal during this. It doesn't sound like it is missing or running rough from the back of the truck. Or even the front or under it. Just from in cab.

I'll see if I can't get a video tomorrow AM.

 

[2004LB7]

I get a thumping sound sometimes on cold starts . I think mine is the hvac damper or something related. Hasn't bothered me much as it goes away after a minute or so

 

[The owl]

That's down the roads of what I was thinking. Or if it's related to transfluid being super cold in the convertor? Like I said, I am sure I'm over thinking this but thought I would ask.

 

[Spank]

I get the same thing on my LMM, cold start and hear a knocking noise. Then goes away after I get close to operating temperature.

 

[NC-smokinlmm]

Load test your batteries.

 

[The owl]

I'll load test the batteries this weekend. I'd assume they are fine, both were replaced last month but weirder things have happened!

Thanks for the replies guys,

 

[2004LB7]

New batteries in mine made no difference

 

[NC-smokinlmm]

My LMM did this when batts were low, cold starts exacerbated it. It would also do wierd things with AC, blow hot on one side and cold on the other bc blend door would get out of range bc of voltage...

 

[TheBac]

Does it sound internal to the engine, or an external source?

 

[motorcare]

Heater Core

My 08 LMM developed a knock just above idle from the dash area shortly after a delete was installed. It was the heater core pipes thru the firewall. I zip-tied the two together to apply a little bind and that was the end of the noise. It was a little hard to find just because the rpm range was so small and it's tough to reach that far back to put hands on it.

 

[The owl]

So now I can't get the truck to replicate this. I guess I'll have to wait for another cold day. In the mean time I finally got my lift pump installed. Which turned into a total nightmare! I'll post a different thread for that one.