Engine rattling noise - can't diagnose, could it be valves?
Here's the scoop. 2.5L, SOHC, 4 cyl. 130,000 miles on rebuilt engine. Engine has plenty of power, issue is not affecting performance or fuel economy. Engine oil was sent out to lab for testing and came back negative/inconclusive (no metal fragments at all) Engine is making an internal rattling noise with acceleration & increase of RPM's...doesn't do it when idling. Timing belt went. (not all the way, just broken teeth) Replaced it. Engine noise still there and maybe a little louder. Change my oil every 3,000 miles, regular maintenance. New plugs & wires after timing belt. Not a belt or pulley, according to mechanic, he checked again....said noise is coming from inside engine and the dealer will want to rip into it to diagnose (expensive!) Friends have told me various things, including to try higher octane fuel (I live at higher altitude) and some kind of engine additive, thinking it might be valve noise. it's pretty loud, I mean you can hear it over the radio sometimes....steady clanking? noise. Any professional thoughts? similar encounters?