A4 Avant power steeering noise

Hi there,
New to this forum and new to owning an Audi. I have just bought a five month old A4 Avant 2.0 170PS. The car came with optional 18" alloys. Occasionally on a hard steering lock to the left at low speed, there is what can be described as a moaning noise coming I think, from the power steering. Before I give the dealer some grief about it, is this a normal noise and has anyone experienced it? Prior to getting the car, two new tyres were fitted to the front but not the same brand as fitted to the rear. The car did not make this noise when I test drove it.
Cheers
ColinP