Just another Idea, on the 117 engines your airpump can make the same sound as the tensioner not holding pressure and it is located right below the tensioner. Not sure if there is the same on a 400e though and it last about 30 to 40 secs. Easy check unplug the clutch power if it stops the noise then it is the air pump.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)