I have a 98 Honda Accord. I hear a humming sound when I accelerate. It usually starts around 20 mph and continues until I slow to speeds below 20. It shifts, but seems to be a little slow to accelerate. Any ideas?

 

Could be tires or a wheel bearing? Have the tires rotated and see if the "sound" moves. Also while the tires are off check the wheel (hub) bearings

 

I had the same problem. Turned out to be the vacuum hose that comes from fuel pressure regulator strings around and over and goes through the intake air resonator was being crimped by the battery sitting on top of it.

Caused a whine/whistle sound like you described

 

Check the wheel bearing, i just replaced a rear wheel bearing on my 98 and also a front wheel bearing on my 02 accord V6.