Help me find my whistle!

Have already posted this in Fabia mk1 section but hopefully someone can help me on here?!

For the past year I have had a frustrating WHOOSH/Whistle on boost from about 3rd gear of full throttle through the gears.

Removed EGR:

Eliminated possible "whoosh" through EGR pipe

I have checked for boost leaks:

Found 1 leak after pressure test: light hissing on joint (bottom pipe, one tooth worn on coupler?)

I bodged this with sealant and for approx 3 days..

Test drove and car felt lively, went well through the gears without a whistle/very faint whoosh for at least 100 miles

3 days later, whoosh returns/less power: (thought sealant could have dispersed/worn away?)

On inspection boost pipe hanging loose:

Replaced bottom boost pipe with second hand one..

Drove ~5 miles: Whoosh returns/less power

On inspection boost pipe hanging loose:

Replaced coupler on bottom boost pipe, fits sound with fresh seals.

Drove ~3 miles: Whoosh/No change/less power

On Inspection boost pipe fitting fine, snug in intercooler joint

Boost test:

No leaks

Tested TWO other Fabia's:

105k BLT Engine 2006: Faint whistle on boost (similar to when I bodged joint with sealant)..lots of power

67k BLT Engine 2007: No whistle on boost, Lots of power

Ok people...what's going on?!!