2000 Questions for all other cars Noise from timing belt in

My Peugeot 206, 1.4 petrol with 60000 miles has undergone a timing belt, t.B. Pulley and water pump change recently. The timing belt was installed correctly and tensioned with facom's dm16 tool to facom's specifications. Since the change a whirring growling noise appeared in the rev band between 2050rpm and 2150rpm. Initially vibrational noises from the t.B. Tensioning pulley were suspected but proven false by a test against the old pulley. The accessory belt and connected units have been eliminated as a source for the noise too. Now the car has run more than 1000 miles since the change with an appropriate tension loss of the t.B. But the noise still has not disappeared.
Another thing that has changed since the new belt was put on is a change in centering of the belt on the pulleys. It is off by about 2 or 3 millimeters (away from the engine). After a try to put the belt back to the initial centering it came back to the off-centered position after some revolutions.
The only thing I have not verified yet is the timing belt itself and the new water pump.
Any ideas on what causes the noise?