2004 Civic Ex.  This problem just started up this week but today it's worse.

Sometimes, when accelerating, I'd hear a hissing noise coming from under the hood, almost like a steam engine.  The noise would only happen when I accelerated and get louder the more the pedal was pushed.  Usually, I could lay off the gas for a second or two, push again, and the noise would be gone and the car would act normally.  Associated with the noise was a noticeable loss of power from the engine, almost like it was straining.  I wasn't able to reproduce it in park or neutral until today.  Today I drove home and could barely get the noise to stop.  It persisted when I parked the car and continued to let the engine run and seemed to be coming from the right side, by the wheel well.  I checked under the hood but there's I can't see anything wrong (though I barely know what I'm looking at).

Thoughts on what this can be/costs to fix/if I can fix it easily myself?  I was hoping to take a 250mi trip this weekend and after today am a bit concerned.

Update:
My CEL's been on for a bit due to some oil issues and I just haven't cleared it, so I went to AutoZone to see if any codes would come up.  Well, I started it up and it immediately started hissing (which is new).  I sat in the driveway and revved it up to 4k a couple of times and then the hissing suddenly stopped and was replaced by a quiet rattle from under the hood.  The rattle's only there when it idles.  Does this help at all?

As for the CEL, it's throwing P0135.

FINAL UPDATE: Old car's toast.  I bought a new used car.  My wallet will recover.

I'm having the exact same problem with my 99 civic ex. Taking it to the shop tomorrow. My friend thinks its a vacuum leak.

hmm. wonder if this is a common thing after 12 or so years....

you mention park and neutral so going to assume its a automatic transmission ... these have some issues Google 2003 Honda civic transmission problems . you also mention the right side do you mean the passenger side?  that is where the trans lives on that car .do you remember if there are any lights on the dash that are blinking when driving or anything like the overdrive light or the d1 d2 lights that are on and you cant get to turn off unless the key is off ?  does the engine idle higher now then before this started ... is the idle different from when it makes the noise vs when it doesn't .. really just from what you said the transmission is not happy and you need to take it to a shop .. might want to tow it there before you make it worse

Plugged cat. Ocassionally the honeycomb matrix that has broken up drops away from the outlet side and it runs normally.

Yes, automatic.  Yes, passenger side.

No excess lights.  CEL is on, but it has been for a bit.  I can always take it to AutoZone to see if it's throwing new codes.  Idle seems fine.

I was really hoping it wouldn't be a transmission issue...

Interdasting...

Update in OP.

P0135 heater circuit failure bank one oxygen sensor.  Sounds like you have some replacement emissions parts in your future along with a possible vacuum leak.

I've been thinking vacuum leak too, but would a vacuum leak be intermittent and cause a rattle?

Get some MAF or carb cleaner. Spray around all vacuum lines - you'll know when you find that right one because the engine will rev.

What are your oil issues? Burning excessive oil will destroy a cat. The rattling you hear may be catalyst matrix that's broken loose.

Slow leak.  Thanks for the vacuum leak tip.

ETA: Obviously, I intend on getting this fixed, my main concern was my weekend trip and if my car's likely to explode or anything (and how much it'll be to fix).

So poking around on Youtube, it should be relatively simple to take the cat off and look in to see if some of the matrix has melted off and is rattling around, right?

If you think taking the cat off of a 13 year old vehicle to check the interior is going to be easy, you are just the person I've been looking for to sell some ocean property in Arizona I own. It's NEVER as easy as it sounds.

Sure simple in theory but rarely in practice, old exhaust systems almost never go easy.  Way too many heat cycles while exposed to the elements and generally poorly maintained.  Have a backup ready when something goes south becase there will be a bolt, stud, nut, manifold, pipe or hanger that keeps you from having a flawless victory, good luck!

That's why I included the words "relatively" and "right?", cause I do not know.

In any case, I decided to go forward with my road trip cause it seemed like I could manage the car ok.  About 90mi in, the hissing started and would not stop.  Temp gauge red lined so I pulled over and let it cool.  Looked up the nearest dealership on my phone and started limping my way there.  Temp gauge would spike after a couple miles and I'd have to pull off and let it cool.  I finally pulled into the service garage and turned the car off and it's steaming and dripping under the radiator.

Dealership gave me $1k trade-in value for it, so I took home a 2013 Civic instead.

Sounded like an easy fix. [new[er] car.]