T4 -99, Weird prob, sounds as if hole in manifold when startin

[swetech]

When she starts up from cold, there is a noise as if there is a hole in the exhaust manifold, however, a few minutes later, the noise goes away (quite sudden) and she sounds normal, whatever valve or similar would cause this ? It's a -99 T4 Petrol 5 cylinder, any ideas anyone ?
Tia
swetech