air whistle after heater core replacement on 89 crown vic

I recently had my 89 crown vic into the local garage. They replaced the heater core but only used weather stripping to seal between the plenum and firewall. I pulled it all apart again and put in the factory rubber and foem seal. My problem is since it was at the garage and after fixing it myself it has a wind whistle at around 100 km and above. I'm perplexed as the whistle will increase in pitch and strength if the air or heat is on and if the fan is switched to high. When turned off the whistle almost completely goes away, but not totally. Can anyone help? Thanks