Shimano BDS Ae Forest DH

 We used our Sony FS700 in the field and then edited it all in Adobe Premiere Pro CC.

 At UK BDS races, there are marshal points where marshals must blow their whistles as riders pass these points on the track. Obviously on practice day, there's rider after rider piling down the track, and marshals are usually within sight and always within hearing distance of each other, so that they can tell if a rider is still on course, etc. Helps with track safety and rider safety. It may be annoying, but there's nothing we can do about it, and we certainly can't edit them out! Trust me, at a race, you get used to it and it kinda helps to to when people are coming too!

 Thanks for explaining, I had no idea that it works like that.
That actually makes a lot of sence

I thought it were just some crazy fans thinking that it will help the riders ride faster
(something like a DH version of the vuvuzela)

 Listen for it next time you watch world cup coverage. Its not just the UK its a UCI wide thing.