I'm tracking down a steady, clicking-like sound coming from the engine bay when driving uphill. The noise is only heard, not felt. It also only happens when towing my work trailer, about 4,000lb. The noise will start off low in volume, then build up loud enough that I can hear it over the radio or with the windows up. While this noise is going on I don't necessarily notice a drop in power, not that there is much power left with a stock 302 pulling two tons uphill. This sound happens regardless of if OD is off or not. There's really only a couple of slight hills in town that I hit regularly at about 40-45 mph. At the start of the hill the clicking noise will start off strong then it'll get a lot lower in volume when it kicks down into 2nd and rev above 2500 or so.

In the last 5k miles I have replaced: plugs, wires, distributor, ignition coil, EGR, MAF, DPFE, PCV, all vacuum lines in engine bay, exhaust manifolds, lower intake manifold, thermostat, radiator, (pretty much the entire cooling system outside of the engine), water pump, motor mounts, transmission fluid, both fuel pumps, fuel filter. Timing is set to 10* and has been checked multiple times. The plugs are not fouled, the wires are not crossed. I recently pulled the throttle body and intake plenum and cleaned them free of carbon buildup. There are no codes, no check engine light has come on during the 6k miles or so that I've owned the truck. I've checked multiple times to make sure nothing is loose in the engine bay. This clicking only happens when pulling my trailer. The noise is still present through tanks of 87, 91, or 93. I have not tried ethanol free however.

I'm assuming this noise is detonation. I haven't tried advancing the timing yet, I've seen in a couple of other threads people suggest advancing it a couple of degrees to about 12*. Anyone have any ideas?

 

Don't assume - diagnose. If there's no reason for detonation, then it's probably NOT detonation. Click this, read it, and the TSBs in its caption:

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From your description, it seems like an exhaust leak to me.Unless you include the full list of reasons (symptoms) you replaced EACH part, along with the diagnostic step it failed, the brand/PN/source for EACH replacement part, and whether it passed the same test the previous one failed... Listing parts you've replaced isn't very informative.Did you compare them to the inside back cover of Haynes?

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They HAVE to cross. But they need to be crossed exactly as shown in this diagram:

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The TB should not be cleaned. It will now need regular & frequent cleaning because you've removed the coating that WAS in it.Then your scanner isn't very good. It should always display a READY code, or a code for incomplete drive cycle, or the fault codes (if there are any).

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Then the bulb is probably burned out or removed. It should prove out every time the key is cycled to RUN.Click this & read the caption:

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