306 GTi-6 - Whats the noise?

[NickyGTi-6]

I have a 306GTi-6 (hence my silly posting name and for the past 2 weeks there has been a strange 'bubbling' sound coming from what seems to be the front part of the exhaust when changing gear in 1st, 2nd and 3rd at reasonable revs. If i take it steady it doesn't do it and has only really been doing it since the weather got warmer. Now before you all cry - take it steady - any ideas what it could be? Have had the exhaust looked at and its not loose, nothing rattling..... crazy!
Please help!!

 

[StealthUK]

The baffles inside the exhaust box may be rattling.