Strange Noise Under the Hood

[Kwing]

So I just finished a 4 day marathon to replace the engine, u-joints, and service the trans in my '07 tahoe. Everything worked perfectly, started on the first try, perfect temp out of the gate, perfect oil pressure, trans temp dead on, engine and trans smooth as silk, BUT...

I have a noise I didn't have before. Kind of a fluttering sound with a hint of click to it. It's a very steady frequency that does not change with engine or vehicle speed. It seems to start about the same time the fans kick on after warm up and will cut out briefly when i go from park to reverse, park to drive, or between reverse and drive. I think that's more a result of circumstance than a relationship to the trans.

It sounds like it's coming from the upper firewall area and is loud enough to hear in the cabin, at speed, but it's not what i'd call loud if that makes any sense. I'm tired, what can i say?

All climate control works perfectly, although on first fire and warm up the heat didn't seem to want to kick on, but second fire up the heat came right on. Not sure it's related, just trying to give as much info as possible.

So any thoughts?

 

[thompsoj22]

Sounds like a successful engine swap, congrats! In regard to the "fluttering" sound, It seems temp related as you state it begins when the cooling fans come on. Any way it could be trapped air in the heater core? I know it is a long shot, is it more pronounced in the cabin or under the hood? Does the sound change with cabin heat fan speed? When the noise starts, Pull the dipstick on the trans and see if there are any bubbles on it also climb under and put a stethoscopelong/screwdriver to the trans pan and see if it is coming from that area. is it deep/shallow pan and was the filter the correct length? im just brain storming here. also really look for the simple harness/hose/line rubbing on the fire wall?

 

[Big Mama]

Kinda the same thought here sounds like heater core or ac box related. I’d also try getting it to temp and turning the heat on full blast that’ll open the blend door and force water through the area.

 

[petethepug]

Exhaust leak comes to mind. Get your smoke tool and disperse it to the area around mani’s. Are pulling any codes? BTW congrats on that install.




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[Kwing]

No codes registered. Nothing i do seems to change the frequency of the sound, including climate blower speed or vent selection (foot/face/recirc/defrost/etc).

Makes the noise whether heat or air is on or off.

Ever heard a dialysis-style fluid pump running either dry or under heavy load? That's the sound. Like a motorboat sound made with "D" sounds instead of "B" sounds.

Still runs like a champ. This truck ain't never run this good . If you have a chance to have a new engine, take it! Just think twice about doing it youself.......

I had considered the possibility of air in the heater core or cooling system somewhere. I would think it would less of a steady-state frequency, but I'd still put it as possible. The noise is definately more defined under the hood than dash.

If it were air in the cooling system,p how do I burp it? Bleeding this cooling system seems to be a black box.

 

[kbuskill]

Park the truck on an incline with the nose up.... the steeper the better... and start the truck and let it get up to operating temperature... leave the cap off the reservoir and add coolant/water as needed.

 

[Doubeleive]

just for kicks take the serpentine belt off and see if the sound goes away or if it comes from the ac pump in which case you can take that belt off as well, this may help identify the problem if not no harm no foul

 

[dmad1]

My Vette has a sound like you are describing. It is the A/C and Heater adjusting to the temp. changes. Will hear more or less on different weatherr days. It is the vacuum modules trying to regulate and maintain a given temp.

 

[Kwing]

So ya'll are gonna laugh at me. I know I did.

The sound is the pulsing of the evaporative emission valve once it opens. Somewhere between hooking up the fuel line and emission line and seating the powertrain on the mounts, that stupid line came disconnected.

Now the fun begins because I can't feel around and find the metal line to reconnect it. Seems it came loose from the holding bracket. From underneath it almost seems like the line is trapped between the bell housing and firewall.

Gonna have to get back there with my scope. But i put a vacuum cap on the valve until i can dig in.

Damn me anyway....