my 87 Honda civic 5spd has developed a loud humming/rumbling sound which starts at around 45 mph and quiets down somewhat around 60. It is unchanged by pushing in the clutch, changing gears, or putting it in neutral. Anyone have any ideas?

 

Do you have any sense about where the sound may be coming from?

 

Sounds like your tires are out of balance.

 

Tire(s) out of balance - very possibly... coulda thrown a wheel weight. Gotta find out where it's coming from. This is definitely going to be narrowed down to one of the wheels. Could be wheel bearings, brakes, half-shafts (fronts only), and even the tires.

 

The fast solution would be to rotate the tires and see if it changes places. That would take half an hour and then you would know if it was the tires or not.

 

Like Windcalmer said, probably just tire balancing, really cheap to get fixed