Check inner fan, if a bearing has gone it can be quite a loud noise.
Otherwise compressor could be cause, check unit is balanced and that compressor bolts that secure it havn't come loose. An unlevel compresssor can also cause clucking noises on start up and shut down etc.
Fan, part number is HUS-SC50-FAN, cost is $49.00, email details below.
Does this sound like your particular issue?
If so, enter your contact details and short message and we will get in touch with you.