Power door goes "Kerbunk" when opening via handle

Hi, the passenger sliding power door makes a loudish "kerbunk" (other descriptions available) like noise when opening via the handle but doesn't when opening via the dashboard button. It opens and closes ok, no sticking. No such sound on closing. The other power door works fine. It is fitted with an after market handle button which doesn't work but I don't think that is the cause. (Series 10 hybrid). It's a bit annoying and I would like to fix it, any ideas? Cheers