strange noises

when my car is at 1500 revs it makes a metallic calanking noise, every now and then it will do it at 2K but it always does it at 1500. can anyone tell me what it might be, im guessing timing because my revs drop and shoot up and thats more then likely a timing issue due to the fact the i have a 125k old timing belt, can anyone please help me indentify this problem though. thanks, this is also my frist post, so im a n00b to this forum.