1999 Honda Accord Ex

Makes whining noise when in reverse and moving or humming like sound. What
is causing problem?

 

it's a straight cut gear, not a beveled planetary gear like other types
of automatic. in simpler terms, it makes the same noise in reverse as a
stick shift.

 

how do I fix it?

 

you dont, really. but if its just started to make this noise, id
probably replace the tranny fluid as a precaution.

 

Let me guess... you just bought the car and it is your first Honda, right?

The sound is normal. Hondas have done that for years.

 

I bought the car new and I have just noticed the noise when I back up.

 

you don't. it's a function of design and afaik, unique to honda.
seriously, it's just like the reverse in a stick shift gearbox.