Good Morning

Recently I noticed a vibration on the steering wheel at 120 KPH.
I had all 4 wheels balanced and the tracking completed but to no avail.
The vibration is still there.
I just happened to be speaking to Aviva ( My Insurance Company) breakdown service guy who explained that the vibration was coming from the power steering belt tensioner.
The belt tensioner feels tight but it may not be tight enough.
Could this be the case.
Has anyone else experienced this.
My car is a 2011 Saab 95 Vector TID manual.

Regards
Denis Ryan
Cork Ireland.