I've recently had the flapper in my Cadet 3 toilet go bad which caused my toilet to constantly fill itself. I figured it was a good idea to replace all the internals (to prevent any other failures) since I was going to be removing the tank anyway... nope, big mistake!

Now I have the fog horn noise (3 second duration) right when the toilet is flushed and it is driving me crazy! Early research indicates it is a bad fill value, but I just replaced that! Could it be defective? I also tried replacing the supply valve and still no luck.Please help end the madness, thanks in advance!

House built in the mid 80's, no PRV installed, water pressure measured at 40 PSI

Toilet: Cadet 3
Replaced manufacture internals with:

Fluidmaster 400LSR Leak Sentry Toilet Tank Fill Valve
Fluidmaster Universal 3 in. Flush Valve Kit

Also replaced original supply valve:

BrassCraft 1/2 in. NOM Comp Inlet x 3/8 in. OD Comp Outlet LL Brass 1/4-Turn Angle Valve

Remove that roller valve on the bowl fill tub and see what happens.

Are you referring to the "water saving" roller clamp? Removed and still no luck... water level in the bowl has increased though. Not much but still noticeable.

There may be some debris in fill valve. Remove cap and flush the fill valve. See instructions.

water level in the bowl has increased though
That's good, means you had it set too low before.

Fog horn sound is definitely debris under the fill valve washer (seal)... that, or there is a tear in the fill valve washer (seal).

Although the seal may simply need to be removed and cleaned (flush water through the fill valve without washer in place), it may also need to be replaced.

Check out this video showing how to replace the seal:

XOlK7X82EUc

Good luck!

Mark