I have an '05 prius with 84,000 mostly highway miles. At EXACTLY 68 MPH and EXACTLY 73-74 MPH (both) I hear what sounds to me the old "engine knock" noise (that's what I call it) I used to hear in an ICE that is operating under strain while going up a hill. It sounds like the noise we were told that a higher grade of gasoline would resolve in a regular gas-only car. It seems to occur only when the ICE is running and both when accelerating and when not accelerating. Accelerating does not seem to make the noise louder. Going up and down hills does not seem to make a difference either...it still occurs. Road surface type (smooth or rough, concrete or blacktop) doesn't seem to make a difference. It seems to be getting louder over the last couple thousand miles (3 weeks or so) I am at a loss. Any thoughts?
Oil level is right isn't it? Sorry I'm not sure but I think you are talking about pinging. Like a rattle rather than a knock? That should only happen under acceleration not slowing down or cruising. You might need a mechanic to go for a ride with you.
To help in figuring out what is causing the noise I would recommend having a person in the passenger seat to help pinpoint where the sound originates. In some cases I have had odd noises that occur in vehicles at certain speeds which were actually transmission noises. In other cases while I only heard them at certain speeds, passengers heard them more of the time. If the noise does not change with acceleration I would say that it is not actually coming from pinging or directly from something occuring in the ICE. It is more likely that a harmonic or the wind is bouncing something in the engine compartment. My first suspect would be the plastic engine cover. Raise the hood and look at the plastic shroud in the front of the engine compartment. If the snaps are loose or missing that is your culprit. (The ICE does affect the cover because when the fan on the engine starts it changes the wind flow under the hood.) I hope that this helps!
