Loud noise between 45 to 60 MPH

[JCHPNC]

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.

[grimple1]

exhaust leak?

[accordspeed306]

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.

[infiniteracing2003]

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.

[Mr. Anderson]

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