Help me figure out what's causing this sound
 

So I've got this sound that just started a few days ago and it's really starting to bother me. It's like the sound of when you have a full air compressor and just drain the tank, that rushing air almost whistling noise.

I'm thinking it may be an exhaust leak but I haven't found any cracks or holes anywhere yet. The reason I think it may be exhaust is it sounds like its coming from behind me and I can't think of anything else back there that would make a sound like that.

The sound picks up and gets louder as I accelerate, most noticeable when I have a wall or something else beside me for it to echo off of while driving. It's most noticeable around 1500-2500 rpm and above that I think the sound of the exhaust drowns it out and below I'm assuming its just not loud enough to for me to pick up on.

The truck is a 98, 2wd, 4.6l, as far as exhaust mods go it includes gotts mod, magnaflow muffler si/do 45'd behind the tires.

Anyone else have any ideas of anything else I should be looking at? I'm open to all suggestions!

Maybe clogged cats?

Didn't think of that, but it would make sense since it does it more once its warmed up. What would be the best way to check those?

Take it to an exhaust shop and get them tested with an infrared tester and/or a backpressure test.

http://www.aa1car.com/library/converter.htm

Check that out, hope it helps.

Hmm I read through that, but I'm not having any of the other symptoms it listed, at least not yet. I have no codes coming up, no drop in fuel mileage, no rough idle, no rotten egg smell, or anything else. The only thing I have is the sound. But I have a birthday coming up in a week, so when I have a little extra cash in case it is a bad cat ill run it by an exhaust shop like glc said and see what they have to say. I guess if its a bad cat it's perfect time to do long tube headers haha.

probably a manifold leaking

Any good way to check that besides putting my head next to it? The old soapy water trick?

If you're under the truck on cold start, you can hover your hand and feel for a pulse of air as you listen for where the sound is coming from, but you have a very short window of time before everything gets way too hot to touch

is it a tick tick tick noise, or a continuous whistle?

Ill definitely give that a try then. And it's continuous sound. Only breaks off when I let off the gas where the sound gets too low to be picked up.

I think jbrew told someone one time to let it cool down, then put the shop vac on blow up the tail pipe. Then go after it with soapy water. That way you will have all the time you need to check the whole system. Might even be a big enough leak that you can hear or feel it with the shop vac putting pressure on the exhaust and not even need the soap.

continuous sound may be a vacuum leak then. because exhaust leaks are more rhythmical tick tick tick tick

If you think it's a vacuum leak, check the PCV elbow behind the intake.

X2 on the PCV elbow