Spark Knock?

Evening everyone! I have a 2007 2.7L Prerunner SR5 Access Cab (manual transmission with about 168K on it. Lots of interstate driving for work.

Recently I've noticed a rattle in the engine under load at interstate speeds (usually if I down shift to pass). I've read some previous posts about different things (piston slap, manifold leaks, etc). The truck has had piston slap on cold mornings since I bought it - so I know all about that. This is more recent. It sounds like bb's rattling around in the engine - had it checked out and shop says the engine is solid - just likely bad gas. I get gas from various stations (usually whatever is there when I'm running low) and where I get my gas doesn't seem to impact it.

Changed out the plugs (OEM Densos), cleaned the throttle body, running good oil, etc. What else could it be? No codes thrown....

Thanks for any advice!

 

I used to get a lot of valve rattle out of my Cherokee when the fuel injectors would start to carbon up. I would usually run it up to about 5k RPM on the highway and wait for the carbon puff out of the tailpipe and then do a can of sea foam in the gas tank.
The valves would rattle like crazy until the puff and then get quiet once it was gone. You can also pull the injectors and clean them and the passageway they fit into. It is a bigger pain but gives you a good idea what is in there.