ST170 engine is the same as any blacktop zetec in regards to timing belt , there is no timing chain

on your picture

1 = air con compressor

2 = top timing belt cover

3 = poly v drive belt

4 = power steering pump

5 = water pump

6 = crank pulley

7 = idler pulley

8 = belt tensioner

9 = alternator

as per the pic one belt drives all these items , you can take it off and run it for a minute or so just to see if the noise goes away and check for movement at air con compressor while its off , if noise is still there then will be timing belt tensioner or idler

john