Hidden items to help you out.
When playing the level "Truth and Reconciliation" (level 3) when you get to the part were the marines say "We'll hang back until you get into postion chief" follow the left side of the wall (stay to the left side of the gunners between the edge of the cliff and the wall) follow it to the end there should be some jackals and an elite, when you get there cortana will say "the chief has the left flank covered" take out the gunners (sniper rifle is your best chance) and between two trees (on the far left side of the mountain) there is a(n) active camoflouge.
It's best to use it when the covenant dropship comes in it cannot see you and it will give you a "Window of Oppertunity" to kill the elites from behind in the narrow passage and move into the grave lift area unseen which gives you the chance to kill all the grunts in the plasma turrents without being seen.
When inside the truth and reconciliation (when you first get inside) to the far back wall there is a health pack and a(n) active camoflouge (best to use right away before the bounty hunter with the sword comes out) or you can wait and use it after the check point (when the big door is locked) and move through unseen by the covenant.
On assualt in the control room when your inside the building (the part where your see the second bounty hunter whith the camoflouged elite and bounty hunter) kill them and under the pillers there is a(n) active camoflouge.
In the library when your fighting the flood there shoud be a ramp (one of the first ramps you see that leads down to a dead end)that leads to a(n) overshield which becomes VERY USEFULL throughout the rest of the level.
There is more items in the other levels but I can't think of them right now, hope I helped.
If you need more help, we've got more Halo: Combat Evolved cheats and also check out all of the answers for this game
Comments for this hint