Loud noise between 45 to 60 MPH

My 91 Accord (104,000 + miles) just starting making a loud noise under the hood - front & center- noise starts at about 45 MPH and stops when you reach 60 MPH. Sounds like a loud single engine airplane. Rides smooth other than the noise - no lights are on in the instrument panel. Mechanic thought it was related to a Code Alarm keyless entry security system (CSI300) recently installed. Install team made sure the siren was extra secure under the hood - they say its not alarm.
Water pump & timing belt replaced at 90,000. Any ideas? Thanks.

exhaust leak?

Yea im sure its a exhaust leak OR it's time to replace your cat. Happen to me but my car was on idle, and it turned out that when i lowered my car i hit my driveways with it and a bolt got loose, so my heat shield was moving.

if its a 5spd, it may be the synchros in that particular gear are going out. iono just throwin out some food for thought. try adding tranny fluid see if it gets quiter.

Does it sound like a truck with really big tires driving down the road???