Boomy soundproofing or exhaust fault or...?

[WILLIAMG]

Does anyone know why I get an excessive boomy broom/broom front exhaust sound for the first 5 to 10 seconds after start up on my 1.4 FSI....it can't be pipe or cat failure, since the noise deminishes and does not repeat-neither is performance affected.

Any ideas anyone? or is it just duff boomy soundprooffing... .?