Hello Iain,
When you turn on the swell tremulant, a tremulant noise sample is triggered, along with HW's tremulant model for each pipe. This trem noise sample also has blower noise, so the organ is programmed to fade out the blower noise sample and fade in the tremulant noise when the trem is turned on.
When the trem is switched off, the trem noise fades out and the blower noise fades back in again.
It wasn't possible to record the trem noise without the blower noise, hence the arrangement described above.
The two noise samples are carefully balanced so the level shouldn't vary too much - however, you can adjust relative levels by using HW's voicing tools, so if you have reduced the trem noise level significantly with the voicing tools (or disabled the rank from loading) you will get the effect you describe.
Whilst investigating this, I did discover a bug - the crossfading between the two noise samples wasn't always happening as it should (there would sometimes be a drop in level after pushing in the tremulant). I've found the reason why and there is an update to correct this here ...
http://www.lavenderaudio.co.uk/oic/Have ... ptwerk.rar
It's approximately a 10MB download.
This bug is present in all the organ definition files and I've just caught it before sending out the first batch of DVDs. As a result, the first lot will go out on Wednesday rather than today as promised - apologies for the slight delay.
David