280 E cardan

My 280 E 1984 manual, has a noise from under the back of my car at speed 80 km/hour up.
The mechanic at lokal garage station told me that was the cardan bearing noise, at if I repair and change the bearing with new set he was still affraid that noise will not gone for the possibility that the cardan gearing was broken too,
He suggested me to change the cardan oil with the SAE 140 from SAE 90 I usually used.
Is the mechanic suggest right, could anybody tell me more better idea to fix my problem
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