With this post and your other one about the tick I'm wondering about an electrical leak in your high tension system made noticeable by your new working electronic ignition.
A better aerial in fringe areas will help the interference as Cobber suggested but that's treating the symptom not the disease. You can't have too big an aerial (unless it starts hitting the trees as you drive past). I remember great big spring loaded ones where the tip hooked to the opposite end of the car and you could release it if you wanted better reception (I grew up in the sticks were the nearest station was over 80 miles away and we would even pull in Chicago, 1500 Km away, on a warm summers evening).
Your rotor and distributor cap must be up to snuff, check inside posts, cracks, tracks, dirt, corrosion that would promote electrical leakage.
Next is the high tension leads. EMP (the interference you hear on the radio) is relieved by resistance in the circuit. This can be as simple as a resistor in the circuit. I had spark plug resistors that plugged in line between the spark plugs and the solid wire spark plug wires on my '71 Javelin back in the day. Just putting in a silicone core lead from distributor to coil will not be enough as that system requires a silicone wire from coil to spark plug (via distributor) to provided enough resistance.
The current technology is metal wound wires. The metal windings inside the silicone sleeve provide a natural resistance to EMP although you don't see that much resistance to the electricity itself. The best of both worlds. The more windings per inch, the more suppression to EMP but the more expensive the kit. You can get a set for the TR7 (Lucas or Delco set ups) from Magnacor for $70. Sprint owners (with their high RPM engines) have been using these for 20 years duration and still going strong. There are a handful of manufactures that make even more sturdy sets for V-8 high horse power high heat engines in the order of over $200 to put the cost of these in perspective. They do not deteriorate with time like a carbon impregnated silicone wire.
A tapping sound caused by leaking electrical with be harder to track down with a mechanics stethoscope than a tapping mechanical mechanic sound cause by cam on fuel pump lever or cam on tappet.
Whether it's mechanical or electrical, can't tell from here but something to consider.
- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT
