even though your trouble is intermittent several things will not change.

while you cannot hear it you still have trouble on the line, just not as much.

go to the endpoint(demarcation usually) and do a full line test by opening the circuit and seeing if are maintaining spec there.

look at your circuit balance and noise.

if you don't meet spec the trouble is between you and the co if it is good the trouble is with the customer or at the demarc.

if you got to go into the central office and test it at the frame do it(for all you know there could be a drier turning on and off in the co.)

keep deviding the circuit in 1/2 and sectionalizing, when you get to that point open the circuit and redo the test.

when your circuit goes out of spec you are close to the problem.

if you are a tech with only 4 yr you probably don't have the knowledge to shoot a bonding problem(star grounds rebonding and fixing defective sheaths by digging up a bad section with backhoes) so talk to you foreman and get hold of your cable tech.

if you are working for a small outfit then congratulations you probably are the cable tech. now you really need to talk with your forman.

the two thing that will never get stolen out of your truck are the shovel and the pick