I will pick up an old corded phone tomorrow and try that in the test socket, if there is noise I will have to contact BT.
Line noise via wifi phone or corded phone should not matter, good news that you heard like a hissing / crackling noise, whilst you was listening, did the tone change of the noise as this can indicate water on the DSL line etc.?
BT responsibility ends (includes) at the master socket, . I picked up a corded phone from Argos (£8) and both wifi and corded phone had same noise.
If you do re-test make sure only corded phone is plugged into phone slot on master socket (if pos) on face plate (not test socket) unless u get bored .
Regarding BT HG612 (B) modem, they do get warm, but not hot, make sure it is not sitting on top anything generating heat and has good ventilation (not in cupboard) – the version (2) got hot but (3) were OK
If noise on line continues, I would plug back in DrayTeK 130 modem and ask BT to test line and to send an Engineer out etc . 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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