Much thanks for the link pazuzu. I had to move a couple of components on the circuit board, but got the needed power supply filter components to fit (I'm using a 1590B enclosure so things are a little tight). This seems to have removed the nasty buzz coming from the external 9v power supply!
Now I have another question. I've noticed that there is a pop coming through my amp when engaging the footswitch. Have searched this forum and found several posts which discuss switch popping issues with other pedal designs, but nothing specific to the beginner npn boost switch project. Is there anything to be done here, did I not find the answer already posted, or is this something that you just learn to live with?