Technical 5000rpm+ and it sounds like a train!

[Ieen]

So whenever my Stilo goes above 5000rpm it makes a loud noise like an old train releasing steam etc.

This only ever happened after getting my head gasket repaired. Before it was 6000rpm it would make this noise, but since it has dropped down to 5000rpm and the noise is made when doing about 100mph in 5th gear.

What could be causing this? I am currently limiting myself from hitting 5000rpm now.

 

[cainy1989]

continually hitting 5-6k isnt good for any part of the engine no matter how well its been looked after, its more than likely coolant/pressure related