Is the radio station AM, FM, TV, or some other service? Interference control puts bread on my table. Do you know the frequency of the radio station? You should, because that determines in part how to reduce or eliminate the interference. How far is your studio from the radio station transmitter? Do you know the call sign of the radio station? If you do, you can look up its frequency, power, etc. on line at www.fcc.gov using their AMQUERY, FMQUERY, TVQUERY, etc. Be aware that FCC regs only protect receivers; they don't protect non-receivers, audio and recording gear, etc. from "blanketing interference" from a nearby transmitter.
If your studio is near a radio station and your equipment demodulates (rectifies) the incoming RF, you should first find the source and frequency. This is the starting point for any RFI solution.
1. Invest in some AC line filters. I use the HUM-X ones. (They can be ordered from B&H in NYC if unobtainable locally for around $60 each.) Note they are rated for 6 amps so don't overload. You should really filter AC to all of your studio gear. Plugging a "clean" mic into a "dirty" preamp or console doesn't fix the problem! You may want to try an "isolation transformer" on the incoming AC to your equipment.
2. Housekeeping: Are your mic cables in good shape (i.e. no floating shields, etc.)? No "ground loops" (AC or RF)?
3. I have used a stock Neumann U67 around broadcast stations with no problem. Some mic preamps ARE a problem. Use a filter on the preamp AC plug also. A good quality shielded 1:1 transformer between the mic and preamp should stop most RF. RF does not like iron. Try the transformer at the preamp end first. Terminate the transformer secondary with a 200 ohm resistor. Ferrite beads on the input of the preamp may help, check the schematic if this has been done.
4. Some mic preamps "dump" the incoming audio from the XLR connector directly into solid state devices, which in turn rectify the RF from the nearby radio station, which you then hear clearly. The input impedance of the preamp may be 1200 ohms or higher. If you are using a U67 (or other tube mic), terminate the output (at the preamp input) with a 200 ohm resistor, so the mic output transformer secondary "sees" the correct load for its turns ratio, especially if using a "unknown" preamp.
5. Are any cables or AC cord lengths approximately 1/4 wave length of the radio station frequency? Your cables can act as an antenna if resonant at the RF frequency.
6. Resolution of "audio rectification" can be a multi-step procedure. Start with housekeeping, then filter the AC, and work from there. Avoid at all costs making modifications to the mic...or connecting capacitors across the mic output. If you want to mess with filters, get some in line shielded "barrel" fittings such as the Switchcraft S3FM type and build in line filters...and LABEL THEM....
I made a remote recording at a chapel where a spectrum analysis revealed a defective MIDI box in a large pipe organ console, which was radiating a strong signal around 2.15 MHz and getting into everything. The organ console was a fixed installation, and the wiring to the pipes and controllers acted like a large antenna. I used multiple mics, an AC isolation transformer and multiple AC line filters, coiling a few turns of mic cable over ferrite core, the whole 9 meters, to get a suitable recording. Moral of the story, other devices with clocking at RF frequencies can also trash a recording.