Do your cars have Vario Cam?
Variable valve timing system Porsche (VARIO CAM)
Advancing the inlet camshaft by means of two sliding chain tensioners, which can be moved to and from via magneto-hydraulic actuation. The exhaust camshaft is driven by the crankshaft. The inlet camshaft is driven by the outlet camshaft via a chain.
Could the resonance your talking about being caused by the cams retarding?