2000 f150 slow to idle down-it takes about 15 seconds to slow down

almost sounds like the cruise control is engaging

There are several things you can do. Make sure the engine is in good tune, replace the air filter, Air the tires up to 34 pounds air pressure to lessen the rolling resistance, How you drive is most important. Take off from traffic light and stop signs slowly. accelerate steady, don't be in a hurry to reach cruising speed, unless, of course, you are merging with fast traffic. Anticipate stops. Start slowing down sooner when coming to a stop sign or traffic signal. Coast on the downhill streets, and use the cruise control as much as possible. Don't waste gas warming up your vehicle in cold weather. It will warm up quickly while you are driving. Any time spent idling is zero miles per gallon.

ignition modules can cause problems like this also airflow meters and faulty fuel pumps

most likey a wrist pin bad or cam bearing and last a rod bearing you need a stethascope to locate it ehrn it's knocking. could even be a pully or belt tensioner.

sounds like restricted fuel filter or restricted exhaust.
your local exhaust shop can perform a simple and free test to see if the cat can is plugged.

IPS Idle position sensor. or IAV needs cleaned Idle Air valve.Spark plugs usually have alot to do with the power of an engine.. Check plugs an wires//

when u disconnected the battery u lost the adaptive idle learn program, do the following to clean the intake and allow normal idle

Here are some of the causes of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

VERY COMMON ISSUE AT THE FORD DEALER I WORK AT, HERE IS WHAT i DO TO FIX IT. THIS WORKS 90% OF THE TIME, IT IS NOT A CURE ALL FOR STALL SLOW IDLE BUT A CLOSE SECOND.
Here are some of the causes of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

The idle issue is normal. The idle is controlled by the ECU. There are no adjustments for the idle. Regarding the knocking noise, that could be several things, and I am hesitant to comment without hearing it. Take it to a Toyota Dealer & have a tech listen to it. We are very good at identifying noises on the service drive.