As far as I can remember, this has been happening for a few summers (possibly since I got the car, but that was a while ago now), but seems a little more pronounced since I switched to Royal Purple for my engine oil. . .
This really only happens when my car has been sitting out in the sun all day. When I'n going up the hill in to Moundhouse, the engine will make a slightly tinny, rapid tapping sound. It only happens at about 25%-40% throttle in 5th gear - enough to hold 60mph going up the hill. If I slow down or give it more gas, the noise stops. If I'm level, it stops.
There's no unusual noise when starting, no noise when driving around town, and when the car is cooler (in the morning,) it doesn't make the noise at all, even when going uphill. I always let the engine warm up before driving, and I've checked the oil - the level is fine, and there's no unusual consumption. The car does not seem to be low on power or doing anything unusual. Gas mileage seems fine. I don't remember this happening in the winter.
I don't really want to think knock. . . I'm wondering if this could be something more simple, like just valve tapping, some weird exhaust leak, or maybe Royal Purple 5w30 just thins out way too much in Nevada summer heat. . .
