I have a 4.6 V8 in my 1998 F-150 with 125K miles. Whenever I put the engine under a descent amount of load, say accelerating normally up hill, or 3/4 throttle or more on flat ground, I get a tapping noise from my engine. It doesn't seem to loose power, but it is a scarry sound. It is definetly coming from the engine bay, and almost sure it is the engine. I did get a check engine light that has since turned off. It was the secondary o2 sensor was bad. I have not replaced it yet, but I also haven't recieved that code again for over 5K miles. Any suggestions on why it would be happening?

 

Is it a mechanical tapping or could it be detonation? Throw a tank of premium gas in and if it goes away, it's detonation.

 

It seems to be mechanical. I've tried putting premium gas in before, and that really didn't change anything.

 

Sounds like an exhaust leak to me.

 

In my truck it sounds like the rockers are rattling / floating. I have read that it could be the retainers. Hope to give a personal report on this problem soon.

 

update: when I went to the gas station today, I decided to try premium gas again. Surprisingly, this has "fixed" the problem. No matter how hard I hit the gas, I wouldn't get any noises. So, I tend to agree with glc that it is probably detonation.
New question: any other areas to look into for the cause of the detonation other than 02 sensors that I will replace the first chance I have?

 

Check for vacuum leaks. Remove the battery and let the comp reset. Replace your spark plugs if you haven't already and use the FACTORY plugs.

If it doesn't fix it, fill up with 89 octane from now on, and call it a day. You'll drive yourself crazy trying to find the cause of that spark knock. Fords are known for it, and I've never seen any problems come of it. But if you spend $.10 extra a gallon, it won't even happen. A $500 service bill will buy you 90,000 miles of 89 octane fuel, if you get 18mpg.

 

It could be a clogged up EGR passage. With 125k that wouldn't surprise me.