E46: Noise When Accelerating

Hi there,
I recently had my E46 328i serviced. Since then, when accelerating from a stop, I hear a whooshing sound at about 10-20 km/h. I didn't notice this before, but my mpg gauge also seems delayed for a couple
of seconds (remains stationary before leaping forward). Other E46 owners: maybe you can tell me if your gauge does the same).
I don't notice a change in the performance of the vehicle, but I do think it shifts out of first gear a bit rougher.
The bigger concern is that this whooshing sound actually changes to a more mechanized rumbling sound after I drive the car for a while. For instance, on two occasions, after driving my car for about 30 minutes, the sound changes! However, after parking the car both times for a few hours, then re-starting, it returns to the whooshing sound when accelerating.
I've brought my car back to the shop that originally serviced it. The shop owner has driven my E46, confirmed he hears the noise, but can't diagnose it and has stated he has checked it all out under the hood and says everything is in order.
Grateful for any feedback you guys have on this...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Check they haven't left the air inlet hose disconnected or similar.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.