Low idle at normal operating temp making a pinging sound--especially louder under load (in drive or reverse with brake applied). No codes. Idles normally when cold without noise.
Any ideas?

 

How many miles on your vehicle? Is it generally well maintained?
"Pinging", as in predetonation?
In addition to your "foot on the brake test", try ascending a hill (loading the engine) to see if you can get it to ping there too.
Believe me, learning how to make the problem recur can be helpful.
I assume you're using the truck daily.

Give it some premium gas.
I'd add a wallop of Techron fuel system cleaner concentrate....especially if you run CGT (Cheapest Gas in Town).
The PEA in Techron is said to clean injector nozzles upon shutdown so add the stuff before local driving not a highway trip.
Many people swear by other chemicals. I've only used Techron.

See if improvement follows. Resume this thread after a coupla hundred miles. Don't expect instant, complete results.

 

Do a quick visual check of KnockSensor Connection\Cable on rightside of IntakeManifold; may be damaged doing other maintenance.