Could be the the aerial seal perishing. Had this on my old van, I just put some sealant around it from the inside and that solved it.
04 plate Ink Blue 1.8 Duratec Ghia. Mods so far, club sticker on rear window and stereo upgrade, 4Q sticker on bootlid and Smurf hanging from rear bumper.
More mods to come as i\'m allowed to.