Not sure if this is relevant but does the LPT have a Dump Valve on the Turbo? if so it could be this that your hearing as it drops the waste boost? if not you have lots of 1/8th bore vac. hoses, check these especialy where they terminate or join, my wifes split a a "Y" joint & exhibited a loss of boost on the turbo/apc gauge.
Regards paul
Regards paul