Stock rear speaker problems

[hadyn]

Sometimes when I start my car I hear a crackling noise coming from my rear drivers side speaker. After about a minute the noise disappears. I get the impression the speaker is knackered as it sometimes fades in and out on that side and also a buzzing noise can sometimes be heard from all speakers which goes when you switch the stereo off.
So how much for a set of replacement speakers to fit the rears and any ideas what causes the crackling and buzz noises? Knackered speaker?