Join Date: Nov 2009
Location: Redford, MI
Bronco Info: '96 Black 302 XLT - '94 Green 351 E. B. both are E4OD, 3.55 axles, 31 10.50x15
A noise like you are describing, that happens ONLY under load is almost always pre-ignition (spark knock).
If your base timing is above 10°, put it back to 10° BTDC and see if that fixes the problem. Do you have any DTCs (trouble codes)?
Another possibility is that you have a very lean mixture, which will will have a tendency to "knock". Large vacuum leaks could lean out the mixture, but will also cause a high idle, and usually some trouble codes.
A third possibility is that you have very low octane fuel. Try a full tank of premium fuel from a gas station you don't usually use (could be that your regular gas station had contaminated fuel).
Also tell us what the last thing you did before this problem occurred (bought fuel/changed vacuum lines/cleaned PCV/ etc. anything, even if you don't think it may have an effect).
"The simplest solution is usually the best solution" - Occam's Razor
"The significant problems we face cannot be solved at the same level of thinking we were at when we created them." - Albert Einstein