Just bought and fitted a new light took down old one that had 3 bulbs new one has 5 just copied old wiring light works but have noticed it humming also the dimmer switch which was fitted with old light also hums is this OK or not

It's pretty common to have some slight humming when using dimmer switches. I wouldn't worry about it .

You may want to just install a new dimmer. Some of the older ones (especially the round rheostat type were very noisy. The newer ones (they look like a standard light switch but they are infinite in movement) are solid state and make no noise.