Radio hissing while speaking; is it interference?

[2Btoo]

My dad has an old MX5 with a radio in it. It's a new(ish) Sony single-din thing, plugged into the car wiring loom and the standard car aerial. And the reception on it isn't that great.

It is mostly used on FM and it can receive some stations very clearly. These sound good, but aren't the ones that my dad wants to listen to.

Radio 4 is the station of choice but can only be found on one frequency in the broadcast range of 92-95FM. And this has a pronounced hiss when there is signal being broadcast (i.e when someone is speaking) but is quiet when there isn't (between sentences, for instance.) I've tried tuning it to other frequencies but it only receives on one, and always displays this problem.

Is this a consequence of interference? If so then why can it receive other stations clearly? There is no other car-related noise; no alternator whine or hum for instance. I'm therefore assuming that there is little point in investigating suppression systems and what not.

In short, what can be done about this?

[TheInternet]

Is the same problem evident on any other radios? (car/home)

[Om]

That just sounds like he lives somewhere with a weaker R4 signal if it is ok on other stations. Perhaps a DAB radio would be an option (assuming there is enough of a signal for that). Or a better aerial?