I have a 2003 civic EX sedan with a manual. It's down on power, pretty significantly. A hill I used to be able to maintain speed or slightly accelerate up now requires downshifting, sometimes two gears (5th to 4th or even 3rd). About 3500 rpm, there is a strange noise. It's hard to describe, it is somewhat like air blowing, but there is another noise too. It sounds like it might be coming from the exhaust manifold.

It doesn't occur immediately, usually after about 5 minutes of driving. It's possible that it is worse the warmer the weather, but I haven't been able to fully test. While maintaining speed, the engine feels like it hesitates/loses power.

I discovered that the air filter housing had two broken "nuts" (the ones imbedded in the plastic of the lower housing. Why do people tighten them so tight?!?!) The lower right corner of the box was not properly seated. I suspect it's been drawing air through there as the air filter looks brand new. I cleaned the housing out and made sure it seats as well as possible, but the original persists.

I'm not sure what else to describe, but I will answer any questions as well as I can. Thanks in advance for your help.