Sentinel: how do you whistle

[john]

So I went through this mission and killed way more than 20 enemies with the grapple but didn't get the Party Crasher achievement. Then I read elsewhere that you have to hide in the bushes and kill them by whistling to get their attention then grapple kill them. Problem is after whistling once (with F I think) I can't whistle again. I'm bedding down in the bushes pretty well too but no whistle. Anyone else encounter this or know a better way to get it done? I'm using KB/M.

[b@man]

Crap like that is why I quit the campaign early on:)

[john]

Well I'm a solo player most of the time. Anyway, got it. The "F" changes to "4" after the first prompt to whistle. For each kill to count you must be in the bushes and get the guards attention with a whistle then fairly quickly grapple him. He must be pulled into the bushes and not someplace else. Too much whistling to get a guards attention gets you game over. No other grapple kills seem to count unless you follow the above formula and by my count if the guard forgets he heard the whistle then grapple killing him will not count either. I guess this is COD's attempt at being stealthy...a game which many other games far excel at.