BMW 528 i SE Touring

[robbieBMW3]

hi i have a bmw 528i its an R Reg
and has 187k on the clock its an imaculate car and runs and sounds like a dream as the 528 always do, but when im doing about 100mph i get a sort of knocking or tapping noice from the engine any ideas, it occasionly does it when its idoling aswell

if you have any ideas of what it is
and how much it could cost to repair any chance you could let me know

thanks

[ab@]

presumably when you're on a track this is?

the easy thing would be to say dont do 100

[robbieBMW3]

no its not on a track

its as if it wont let you go much higher than 100mph