I have a 2003 Civic DX manual 5 speed with 346,000 miles. Everything is stock, no mods, daily driver, runs great and still get 35-40 mpg.

Been noticing when the outside temps are around 30 or less my car has developed a slight whistle at different high speeds. Crappy thing is, even though it is a manual I don't have a tach so can't specify rpm. The whistle is only present when giving a light throttle, not when idle or accelerating, only when maintaining a high speed. For example, the other day it was in the 20's, cruising along and the whistle started around 68mph, let off the gas, it went away, brought it back to 68 it came back so then I accelerated, it went away. Took it back down to 68, maintained the speed then it came back. This morning was similar yet the whistle was present around 60 so it sort of varies. When hearing the noise I usually turn off the radio and vents so I can rule out A/C or radio noise. I even bent back the wear sensors on the brake pads just to verify it isn't them. I still have plenty meat on the pads but wanted to rule that out. Last night I took apart the complete intake, cleaned out, cleaned the throttle body, checked for vacuum leaks and all looked good. This morning, we were whistling again. During the warmer temps this is not present.

One last note, I can be driving 80 for about 45 minutes without hearing it but once I dial it down around 60 to 70 it shows its ugly self. Any suggestions would be appreciated.

 

wild guess here.. but could it be the belts??

 

Thanks for the wild guess. I don't think it is the belts but I will take a further look at them. Tension seems to be fine but I didn't look too hard at them. Doesn't sound like a normal belt squeal but I will definitely check.

When driving with my wife I asked her where she thought the sound came from and she said the drivers side, yet from the drivers seat I thought it was coming from the passenger side. It was about 25 degrees this morning (a little warmer than the last couple of days), the whistle came on around 60mph then never heard it again. This is just a freakin mystery, idle is nice and smooth, acceleration and power are fine. Might do some vacuum tests in the morning when it is coldest since it seems like it has to do with outside temps....what the heck.

 

Couldn't find any vacuum leaks this morning. During the drive to work the temps were in the 30s, no whistle'n.

 

Think I got it, but not 100% convinced though. Due to not being able to find anything I decided to replace the easiest thing, the air filter. Again, this car is totally stock so nothing performance on the car, same ol paper filter...and yes, it was a fram. I didn't see anything wrong with it but I didn't go over it with a fine toothed comb either. Since the replacement it has been cold and I haven't heard anything. This would be a first.