Probably spark knock because of EGR system fault or #1 main bearing knock. The both are harmless.My mother has a 1995 Sedan Deville. The last year it has been making noise under the hood that we just can't explain. We thought it was gas related and tried both medium and high test grade fuels and fuel/engine cleaners. We have tried mechanics who don't have a clue. At this point, I think it actually is something under the hood. I realize this is the hardest thing to figure out over the net, but I'm willing to give it a try if any of you will.
The sound is only when we give it gas. It reminds me of a head sticking...like a tap-it. Don't know if that's how it's spelled. Does this sound familiar to anyone?
Thanks to anyone who might have a clue.
1. Is the noise is more pronounced when engine is hot and at light throttle. Say, while driving around a block (10-20 MPH) with hot engine (180F plus) ? If so it is most likely plugged with carbon EGR tubes inside of throttle body, a bad EGR valve or/and a bad EGR solenoid, or vacuum leak caused with cracked vacuum tubes (most likely ones around the throttle body).
2. To eliminate the bearing knock decrease the tension on the serpentine belt by turning the belt tensioner with a 1/2" wrench or a bar while engine is running (be careful). If the noise goes away it is the bearing knock. If you use a thicker oil say Rotella 15W-40 the noise will be reduced.
3. Another possibility is lifters.