Slow humming sound after spark plugs changed

[TXScott]

Just changed all my spark plugs and the PCV. Not my first rodeo, gapped the plugs to spec(they were pretty much there already), coil packs looked fine, replaced the air plenum gasket, put on anti-seize lube on the plugs after I was burned owning a Mustang where they broke off inside during a change. Anyway, when I start up now, still in Park, there is a low hum that comes and goes like I have some sort of alien reactor technology around .3 Hz. Since I had to remove the air plenum as part of the replacement, at first I thought bad vacuum hose connection when reattaching, but they all seem snug and no hissing sound. I used all Ford parts. Any ideas what's causing the slow, ominous humming?

 

[peterk9]

Welcome to the Forum.
Just curious if you also get the hum in accessory mode without the engine running?

Peter

 

[TXScott]

Welcome to the Forum.
Just curious if you also get the hum in accessory mode without the engine running?

Peter

No, it's only present when the engine is started. I couldn't find my ForScan ODB-II dongle so I ordered a new one which will arrive on Friday. I used a dongle off my wife's Kia, which is much more basic, but it didn't show any codes.

When I drive it around, it seems fine, even a slight bit more peppy since it was nearing 100k and needed those new spark plugs.

 

[Explorer75]

Just changed all my spark plugs and the PCV. Not my first rodeo, gapped the plugs to spec(they were pretty much there already), coil packs looked fine, replaced the air plenum gasket, put on anti-seize lube on the plugs after I was burned owning a Mustang where they broke off inside during a change. Anyway, when I start up now, still in Park, there is a low hum that comes and goes like I have some sort of alien reactor technology around .3 Hz. Since I had to remove the air plenum as part of the replacement, at first I thought bad vacuum hose connection when reattaching, but they all seem snug and no hissing sound. I used all Ford parts. Any ideas what's causing the slow, ominous humming?

Did you re-install the intake manifold using the following sequence?

There´s also a bolt underneath that maybe you forgot to torque it. I believe it´s this bolt:

 

[Gary Crist]

I hate to ask but is that .3 or 1/3 HZ as in as infrasonic?

 

[Mbrooks420]

I hate to ask but is that .3 or 1/3 HZ as in as infrasonic?

Indeed. You can’t remotely hear anything nearly that low.

Did you use the exact match on the plugs with what came out? Are you sure you didn’t change the routing of the wires?

 

[TXScott]

Did you re-install the intake manifold using the following sequence? View attachment 327892

There´s also a bolt underneath that maybe you forgot to torque it. I believe it´s this bolt:

View attachment 327893

Thank you for your input, I will need to try this. I have one of those digital torque gauges that fits between the ratchet and socket but it was dead even after changing the coin battery so I just went with ye olde "tight+1/4 turn" when putting the air plenum back on(with a new gasket) and the new plugs. I will get a torque wrench and see if correct tightening helps.

Also, got the ODB-II dongle for use with FORScan in the mail today and hooked it up but there were no real problems detected.

FWIW - some CPU's and other FRU chips use this ordered tightening sequence method as well

 

[TXScott]

Indeed. You can’t remotely hear anything nearly that low.

Did you use the exact match on the plugs with what came out? Are you sure you didn’t change the routing of the wires?

Hz can also used to measure frequency in terms of time intervals, or in this case about once ever 3 seconds.

I changed the plugs one at a time so as not to worry about wiring mishaps, altho tbh there is much excess wire off the harness to get them confused. I used the stock Motorcraft platinum plugs(part numbers matched old plugs) and even bought them direct from Ford so as not to worry about scammers on eBay or Amazon selling cleaned/used or outright fake parts.