You are experiencing a problem that can be solved easily, using techniques
known by radio amateurs. In particular, The Radio Handbook, bu Bill Orr,
has a chart showing the attenuation of signal levels by various bypassing
methods used at the case of a shielded piece of equipment.

At 100 MHz, the attenuation of a standard bypass capacitor is not very high.
Additional filtering will reduce the level to where you cannot hear the
noise coming from the microprocessor. There are special capacitors, called
feedthrough capacitors, which do a great job of attenuating the signal
level, and that combined with a pi-network will result in no signal coming.

I would suggest that you borrow a high bandwidth scope (200 to 400 MHz) to
look at the levels coming out.

Where ultimate attenuation is required, double shielding is often used,
however I do not think you need that level of attenuation.

Colin N12HS


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