2011 f150 5.0 175k
So originally thought I was developing the known piston slap issue. But after reading around and due to noticing other circumstances around the issue, I'm wondering if Im having a fuel or ignition issue.

In higher rpms, say I'm going 20 or even 50, and accelerate heavily (not WOT dont want to damage anything) enough to make the trans shift down and rpms hit 3500+ id say I get a rapping noise from the engine. Like rod knock, but more like piston slap. Never had a dead cylinder/misfire issue like this so I'm not sure what it would sound like.

Recently I accidentally hit the tow button on the shifter a couple times. Slowed down in a curve going maybe 30-40, and when I accelerate gently to moderately I get the same noise. Noticed tow mode so got out of it, feel trans shift, and then no issue. Note that I was not towing. Towing my single axle trailer with 3 riders on it doesn't cause the noise aside from the heavy acceleration which I'm careful not to do. I don't use the tow mode in this case either due to the above mentioned scenario.

Makes me think in tow mode the shift points are different and causing a misfire.

 

if it was a rod knock, quckly smapping the throttle very agressively willl make it knock sitting in your driveway in park.....
get the rpms up to like 2k-2.5k & then quckly snap the throttle to like 4k
quickly on & off, over & over.
rpm range isnt super inportant, higher is better.

Do this after the negine has warmed up some so the bearing clearances have enlarged from heat as they always do.

My guess is it could very well be a rod knock,

 

I considered that too. Needed to replace the oil pan gasket, so figured I'd check out the rod bearings and Journals while I was in there. Journals and bearings looked very good. No scratches, grooves, heat spots. Bearings were faintly showing brass in areas, so I replaced them.

The tow mode is was has be second guessing piston slap or misfiring. I'm in low rpms and light acceleration, and getting the noise in tow mode.

 

i think you found your issue...
bearings dont just show brass but crank is fine usually, there will be taper, etc, that only micrometers will show.
bearings dont just show brass for no reason.

days of the backyard rebuilds are over....


do as i said, get it hot & rap on the throttle....

easier to isolate standing still vs whats that noise drivinng diwn the road,,,,

but then again, you say low rpms...

take a video if you can reprodue it at a standstill as i described, event if its a lower rpm range.