Have you tried selecting a station that does not broadcast RDS? The RDS signal is outside the audio bandwidth (it's actually at 58kHz or three times the 19kHz Stereo Pilot Tone that allows your receiver to determine left and right channels). All you do when selecting it on is turn the decoder on in the radio, but that should not affect the audio demodulation at all.

Is the radio dropping out of stereo into mono and that fast signal switching is causing the clipping? Most radios will drop to mono in weak signal areas as a way of keeping at least some audio going.

Does it do it on all radio channels?

Do you use EON (Enhanced Other Network) which will switch from transmitter to transmitter as you change service areas - assuming you have very wide area coverage or national coverage networks. EON signal switching in two weak areas could do it as the radio changes receivers (EON uses 2). Europe invented RDS and RDS EON but I don't know how much has been implemented and what 'modes' made available to you hence the possibly strange questions.

If not then look for aerial connections to be clean and all the plugs tight as you don't report the issue on any other audio source.

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