Having an issue with something causing a high RPM noise. I have just installed an RPM gauge and during this time at 60+MPH the RPM registers at 2000 RPM, but the sound seems like it would be closer to 5-6000 RPMs. during idle the rpm gauge sit around at 1000 RPMs. I installed the overdrive mod from IPD last year and before that had put in a new overdrive relay, but have never had this noise before. I'm not completely sure it's the engine as I can only get it to do this on the highway, and I don't want to be doing this very long at a time as it doesn't sound right. So I can't really take any highways at the moment. as limit is 60MPH but everyone here in Florida is going 75MPH. I'd like to go 75MPH but at 65MPH the sound is very worrying. any ideas as to where this is coming from? It does sound like it's coming from the front, which IMO would be either wheels or engine, but I don't know why or how it could be the engine when the OEM RPM gauge. (the indash gauge where the clock was) is reading 2K RPM. thanks!

 

FYI, an 'rpm gauge' is called a Tachometer

I think the first thing I'd do is verify that in fact your tach is reading the right RPMs. this could be done with a automobile-specific test meter that has a tach setting (most garages should have one of these).... at idle, both tachs should read around 700RPM, and you can briefly rev the engine to, say, 3000 rpm in neutral/park, and verify your tach read approximately the same as the multi-meter.

I'd say disable your OD while going 60MPH and see if this sound you're hearing changes (the engine should go from ~ 2000 rpm to about 3500 or something, so you'd hear that anyways), but your OD mod/repair disables the ability to switch the OD off.

what kind of condition is your automatic transmission fluid in? its possible the transmission is slipping at higher speeds, this would result in much higher RPMs, but these should register on your tach.

ah, I know a test you can do. go ahead and get up to 75 or 80 on a empty remote stretch of highway, and slip the car into N(eutral) and take your foot off the gas and coast for a bit.. the RPMs should drop right to idle, while your car slows down relatively slowly.... if you still hear the noise, its drive shaft or transmission or wheel/tire related ...