AAhhgg, more options. Thanks for your input tho. I like the idea of replacing the L297 with a pic as this is apparently the main cause of the buzzing. but way beyond my capabilities at the mo.

Ive been looking at the follow up thread to the A3986 debate (big can of worms) and it seems that PWM is better than the chopper control of the L297. However there was also a comment on a similar theme that suggested that the L6203 isnt the best choice for the h bridge either

Looks like I'm going round in circles.

Steve
I believe the pic is lowering vref only when the step signal stops. In normal use the trim pot is setting the Vref.I agree that the 555 option is the easiest but the pic option uses less components so helping with a smaller board and slightly reduced costs.

Thanks again