Bent pushrod on a 455 big block engine causing rattling- could the problem be a loose rocker arm or wrong size of pushrod? What causes a bent pushrod? and iam ****

the high lift cam has no compensation and so the valve retainer bottomed out causing the rocker to slip sideways and bend the push rod
you would have been better fitting a prolonged duration cam rather that a high lift for the following reasons
1 --- you can used standard rocker gear
2 ---there is no valve bounce at high rpm
3 --- smoother air flow in the inlet manifold so there is not such a stop/ start action when rpm is low
4--- less wear on cam followers and cam lobes
cam shafts are all about open duration so longer open with smaller lift is better than the same duration with a fast up and down action

I would try an engine flush and oil change first. It probably has a hydraulic lifter that is not pumping up due to low oil viscosity, or dirt in lifter. If that don't work then you may have a rocker arm coming loose, or a bent pushrod.

most common ticking noise under the hood is from the valve train [burnt,bent,sludged pushrods]--burnt,bent,damaged valves---[loose,damaged rocker arms]--burnt,flat spot,sludged camshaft----sludge build up in engine-low oil pressure ---need oil change-other can be bad idler pulley,alternator bearing going bad

It could possibly be a bent connecting rod, a bent / broken pushrod, broken spring, loose rocker or even a loose nut...

Hope this helps.....

Determining what caused sudden slack in valves would be of concern to me, as in perhaps, changed oil brand, grade or filter incorrect?
Most small block chevy's have adjustable rocker arms. Is that not the case with yours?

This is usually caused by a bad valve or a bent pushrod. Either way at least one of the exhaust valves is not seating properly. You could pull the valve covers, remove the rocker arms, and pull each pushrod and check them one by one for straightness to eliminate the easy fix, but if you don't find a bent one you will have to pull the heads to check the valves. Personally I would pull the heads anyway and inspect them because a bent pushrod could be a symptom of a bigger problem.

A rocker arm is the arm that sits on top of the valve stem and connects to the push rods 'the push rods sit on the cam so when engine is running, the push rods go up and down in a timed sequence and with the rocker arms, open and close the inlet and exhaust valves. the rocker arm is located inside the valve(rocker)cover .If the rocker arm gets loose or comes out.
Check the nuts on the shaft which holds the rocker arms.either the shaft or nuts are got loose.They need to be tightened.Don't make it too tight or too loose.  Too tight,and they don't flutter right. Too loose,and they allow the rocker arms to wiggle out of whack. Other possibility can be  something is broken, bent, collapsed, or out of adjustment. 
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He meant bent valve. you are looking at a head job. sorry but these are interference engines.
In fact I believe it has an overhead cam right? If so then there are no pushrods. Generally all a bent pushrod does is leave the valve closed.

Loose rocker, bent rocker, bent push rod, loose or bent rocker stud are a few things that come to mind. Take off the valve covers and with each cylinder at top dead center, check the play on each rocker assembly and inspect for any bent parts. Visually look down, and to the side for any mis-alignment.

check oil pressure possible valve tapping -start vehicle and open hood if the noise is on top and sounds like metal tapping u may have a problem with a bad lifter ,oil pressure not lubing head,bent valve/push rod,loose rocker arm. if noise towards front check belts and pulleys-power steering level