Whining/grinding noise when acceleration/deceleration

Hello

The car is 1997 LS MX-6 V6 5-speed.

I'm fighting with this problem since I got the car.
When I'm driving I hear a very annoying high pitch whining/grinding noise.
It is not related to the clutch. I tested it and when I press the clutch or not the noise is there.
When I'm around 3.5k-4k RPM it's most noticable and very very high pitch. After 5k RPM you can still hear it, but not so much..
I can hear it at idle in neutral aswell. I can hear it in every gear.
I have near stock air filter system, what I changed that it has a K&N Filter instead of OEM one.
Cat-back exhaust with headers, but I had this problem before I did the modifications.
I hear a hissing on the left side (standing in front of the car, where the timing belt is).
It's like coming from the right wheel (passenger side).

Can anyone help me with this? I can't find a solution and I tried a lot of things already.

Videos below: