Rattle at 4000 constant reves

there is a lead on the gearbox that dose ur speed counter this mite be the same lead as the reve counter

run fault codes and check for MAF/MAP sensors and HO2S and HO2 sensors.

we really need more info...type of vehicle...type and age of fuel..type of fuel system.... so trying to hold the revs at a constant over 2000 rpm and it hunts? or revving the throttle on and off it smothers?

The fuel filter is probably plugged.

you are trying to go higher then the rev limiter its normal operation

Check for a vacuum hose leak

this is your rev limiter stopping you from over reving engine causing internal damage. this is normal on all new vehicles. Don't know what year they started this but it goes back at least 16 yrs

THE RATTLING YOU HEAR AT LOW R'S IS MORE THAN LIKELY THE CAM CHAIN RATTLING. THE CHAIN TENSIONER IS ADJUSTABLE TO A DEGREE BUT A BIKE OF THIS AGE(IF OVER SAY 10000 MILES) MAY NEED A NEW CHAIN TO ALLEVIATE THIS PROBLEM. LOOSE CHAIN CAN CAUSE CAM TIMING TO VARIATE CAUSING A SLIGHT MISFIRE.
THIS MISFIRE COULD ALSO BE THE RESULT OF DIRTY CARBS. IF SO THEY WILL NEED CLEANING AND SYNCING.

My engine light is on in my 199 ford explorer and I was told it is the o2 sensor 1 and 2 what does this mean?

The revs were reset wrong, get them re adjusted. Go back to same garage and ask for it to be done free, as it was done wrong first time.