Alright, i had this prob since the i bought my mustang, When im in 5th gear cursing about 80 mph on the iterstate and when i go to pass someone or when i go to give it a little gas i hear a little knocking, It only does it when im in a low gear. Im told its spark knocking, im running my car plus gas, but when i put premium in it wont make the noise, So how can i fix this prob? new plugs, boots, and coil packs?

 

the noise you hear is detonation. to fix this, you can either run a better grade of fuel, switch to a colder sparkplug or have someone pull timming, as a last resort

 

Can someone kinda describe what detonation sounds like? Is it usually only heard under load?

 

From what I hear, it sounds like marbles in a tin can and yes, it usually happens under load.

 

try some new spark plugs, change fuel filter, and run a can of seafoam through the gas tank.

 

detonation is easier to feel than it is to hear (at least for my deaf ***). You'll know it when you suffer it. Feels almost like a car that's missing or running out of gas. I've always felt a definite stumble. The sound is a lot like a diesel which isn't surprising since a diesel's principle of operation is detonation.

You need to have your tune checked. It's possible that the AF is off and 91 octane is only quieting the problem instead of fixing it. Lean AF ratios result high EGT's and that can damage pistons, valve train pieces and catalytic converters.