Mystery... Pure Hum @ 1400-1800RPM only. WTF?

After replacement of 1) valve cover gasket, 2) entire exhaust piping.
Kept/reattached Oxygen sensor at engine end with weld. Done at "good" repair franchise location. Original V8 4.7L engine.

Immediately noticed low-freq hum. Very pure & steady, deep baritone.
Only when applying gas & @ 1.4-1.8K RPM.
Doesn't happen when throttling up in Park.
Only happens after engine temperature goes above "cold", above the bottom gauge mark.
Repair place technician said it seems normal. It's not.

theories:
* Tranny issue & just coincident with recent work? Fluid pan not changed for over 80K miles. No other issue shifting.
* Resonant freq. of loose gasket or exhaust manifold or exhaust pipes?

 

Check wheel bearings? Exhaust heat shields?

 

I’m going with exhaust hum or drone. Maybe a hanger, heat shield or connection.

 

Did you use OEM resonator and muffler?

 

My eyes would be firmly centered on the exhaust. Did they touch any flanges? Did they take anything out? Did they add anything in? Did they relocate or move anything?

If this is a "Good" franchise/chain as you say, they'll be more than willing to go on a test drive with you to hear you reproduce the problem, and look into it. The problem didn't exist prior, exhaust is notorious for 'droning' and other acoustics. It stands to reason it's something they did.

 

Wheel: But it doesn't occur without throttle up, i.e. coasting - where RPM=0. Could be shield.

 

New muffler & resonator through Meineke.

 

Thanks. -- Forgot to say: no change in engine power/performance.
They did test drive w/o me. Claimed that they can't hear anything abnormal.
I'll check w/ dealer/other to use stethoscope type device to pinpoint the source, but that's gonna be hard to do, given the requirements to reproduce the noise.

 

It's definitely something resonating, and resonating in response to throttle, it sounds like. You may very well have an exhaust leak also.

 

I would agree with resonator for sure.

 

Resonator might not be tuned into the right frequency to eliminate drone since it’s aftermarket.

 

Are the stock resonators tiny glasspacks?

 

A glass pack is a basic resonator but not all resonators are glass packs.

 

uh.... dunno.

 

Would you say it's like this?

https://youtu.be/XYVXOky-j5g?si=TaAdn_jRZpDucaax