Engine noise

My 2.25 petrol engine is making a loud clattering noise that increases with the revs. This only happens when the engine is under load ie reversing back into the garage. I've taken the belts off so not bearings. Fairly smoky as well. Ticks over ok , a little lumpy. I've taken rocker cover off and used my starter to turn the engine over. All the valves seem ok . Any ideas anyone?

Have you checked the valve clearances?
Any of the rockers kissing the underside of the rocker cover?
Checked for wear on the rockers where they strike the top of the valve stem?

To be sure, use a dial indicator rather than feeler gauges. Hollows worn into the rockers can give a false reading with a shim.