how to identify fault on my Creda 37400 tumble dryer?

On switching on the machine the drum will not rotate , it simply hums ,so I switch off immediately . With the outer casing removed , the drum feels stiff to turn it round , buy as soon as it is moved the motor instantly starts the drum spinning as normal . The dryer is 34 years old but has been working perfectly until now . How do I check which component is faulty e.g. rear drum seal , front felt bearing , motor ? Any guidance would be much appreciated - thank you .