I saw Buster's post this morning and wondered if he had come up with any answers yet. We have similar issues perhaps.
This week my son bought a repossessed 2006 Mazda 3, 2.3 hatchback 5 speed with 95,000 miles from an auction. The previous owner had over-sized the tires and modified the air filter and the exhaust systems. I am still trying to figure out exactly what was done, but the filter looks like a "K&N" brand canister type, and the exhaust tip is a "3R Racing" product. I don't know if it is just a tip or if the whole exhaust system was replaced. The sound it makes is just ridiculous. Although the whole "Tokyo Drift" thing is not his scene, he could live with the modifications if they are not causing problems. The car came at a good price even considering the immediate repairs that obviously needed to made (leaking front strut, and a burnt out master switch for the power windows.)
My son's car exhibits the same symptoms as Buster's. In addition, the noise gets considerably louder and faster when letting out the clutch going up hill from a dead stop. It is pretty intimidating. The intensity of the sound gave us the first indication that there may be a more serious issue than just a valve in need of adjustment. Call it a flutter, tap, click or a clack; it is likely a valve noise and hopefully not a rod bearing going bad. It idles fine...the noise is barely, if at all, noticeable. The transmission does not seem to be an issue, it shifts smoothly and if taken out of gear, the flutter can be duplicated when the RPMs are put in the 1800 to 2200 range.
I wonder if it could be related to the exhaust work causing some unwanted pressure change. I have also looked all over the net for information on adjusting the valves and have yet to find a clear answer. Some sources say they can be shimmed, other say that the only adjustment is to replace the lifters.
For first steps before we do some expensive poking around, I added Lucas upper cylinder lubricant to his first tank of gas and I'm going to add a half quart of Marvel to the very dirty oil. He'll drive it a bit before we do an oil change and replace the PCV and see if it responds at all. Any one out there who can offer some additional knowledge experiences, or troubleshooting thoughts for potential causes? Thanks.
