I've been trying to track down this problem. On moderate acceleration such as an "on ramp", the engine has a low pitch sound then when hitting about 3,000 RPM the engine pitch changes to a more normal sound and it feels like a gain in HP kicks in. The low pitch sound feels like a dirty filter but I have a K&N that is fine.

I've tried the following over the last few months that I would think may affect the problem:

I have a bad air filer box so I suspected air leaking, so I taped around the split
About a two weeks ago the knock sensors were changed due to random P0330 codes
Spark plugs were changed back to OEM Denso IR
IACV has been cleaned

 

You might want to try it without an air filter for a test, to see if if you note any change in sound.

 

Very nice! Took out the filter and this baby has never run better. So quiet and smooth on acceleration.

 

I cleaned the K&N about 4 months ago. Maybe it's time to trash it and get a Wix. Seems like some people don't like K&N and have complications.

 

I kinda liked my K&N drop in filter,but it was always infecting my m.a.s sensor. Plus I lost a tadd of off the line performance. So I put a purelator (factory style) filter back in and never looked back. Reset the ecm of course too.