Help !!!

Power steering pump is working fine as is the water pump and the belt is in good condition However on starting the car there is a nasty sort of metal grinding sound that lasts for 3 or 4 seconds and then goes away ( can get it to last longer if I hold the key when the engine has started )...I suspect that the Drive belt Tensioner pulley is the cause and want to replace it...I need to know if I can just purchase the Tensioner Pulley and replace it with ease... eg can I take the belt off and unbolt the Tensioner replace it and put the belt back on ? or is the task a nightmare.... Your adive would be Very Very welcome... Thankyou for taking the time to Read ... Russell.