2006 Ford F-150 detonation

Truck developed a noise that my local mechanic says is due to "detonation" (aka spark knock/pre-ignition). I was pulling my pop-up trailer (3250lbs) with a full load in the bed (about 500lbs) home and at about 40+ mph I heard a sound on the drivers side that sounded like a combination of an air-hose leaking and a tree branch dragging). Once I got to 50 mph it stopped. However, as I begin climbing a hill it starts again. It seems to happen when the engine is "under load". I have pulled my trailer many times and never had this issue, even with "over-drive" engaged, A/C running and cruise-control set to 70mph. Usually I can't tell I am even pulling a trailer.

I was able to recreate the sound without the trailer, and it starts around 45+ mph. If I accelerate slightly after it goes into over-drive I get the noise. It happens at all speeds. I don't have any loss of power, just the noise. What causes this? My local mechanic said it might be due to "bad gas", put I put in 89 octane and it is still doing it. I don't want to spend a lot of money at the dealership and still have this problem (I don't put much faith in dealerships).

Any ideas? I need to get this resolved soon, because I have another trip planned it a few weeks.