Gas Tank Whining noise

[Big Dirty]

I have a 1981 Chevy 3/4 ton camper special, 350 5.7 4 barrel quadrajet with duel fuel tanks.

I just recently replaced the fuel pump and after doing so I I now have a high pitched whistle noise coming from behind the cab. It sounded like it was coming out of the gas cap so I removed one of the caps and the sound stopped. To be honest it's almost like the whistling is due to excess pressure in the system being pushed by the cap and making a whistling sound. It doesn't matter which cap I remove (left or right tank) the sound will subside, but then start again when I put the cap back on.

Is this new fuel pump pressurizing the system too much? Should I buy an after market cap with a pressure blow off?

Any help would be appreciated. Thank you.