Car radio has a lisp???

[vanordinaire]

I listen to a lot of radio 4 while driving for work, so mostly spoken rather than music programs. Recently I noticed that a great many of the contributors and presenters seemed to be talking with a lisp then I realised it was only when driving one of my cars in particular. It seems that the radio is cutting out many of the 'ssss' sounds leaving the speakers with a speech deficiency, over pronouncing their t's and th's. It was amusing at first but I'd like to sort it , any suggestions?