you say it goes when the clutch is pressed, but that means its possible for you to be doing 120mph but if you press the clutch the engine could be idling.
If you can sit with the car stationary, in neutral, clutch pressed and the noise is present when you rev the engine like this then your noise is engine related.
if when you let the clutch out and rev the engine in neutral you've probably got a clutch / flywheel issue (that's my favourite guess right now), or if the noise only occurs whilst driving / under load then its going to be transmission / drive line related.
The 20v JTD is known for eating its drive shafts.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso