I got this Accord and I thought it was a vacuum leak making the noise. I began to spray around and see if I could locate it but I couldn't find anything. The sound literally sounds almost like a turbo spooling *I wish it was*. It only happens when the car is driving, when in neutral I can't reproduce the sound by moving the throttle cable.

My engine bay has been hacked to heck by the previous owner, as you'll see lots of electrical tape in the picture but I found a hose that seems to be attached to the transmission but the other end is just hanging out. I wondered if the transmission has any kind of venting hose or a vacuum line that this could be? I attached a picture of the hose in question. I looked for a vacuum line diagram online and this didn't seem to match. The motor has had power steering removed and the AC removed.



The engine has had the timing belt & water pump replaced. And the sound seems to come from the passenger side of the engine bay, which is where the hose happens to be located.

 

That's just a vent hose for the transmission, don't worry about it. It sounds like you've got a small air leak somewhere that you can't find. I would run down to an auto shop and ask to use their EVAP tester to smoke out the vacuum leak. It'll only take them about five minutes to do, so it shouldn't cost very much.

 

It looks like you have an intake on the car. Often times air moving through the throttle body can creat a whistling sound. Intakes make it very noticable. Make a video, I have a feeling thats what this is.

 

it seems to be mostly once it warms up. not at cold or idle. could it be a belt or alternator? the car has problems with electrical load like rolling the windows down when I start it. the motor rpms drop to only a few hundred and it about stalls