Rubber tree indoors having great leave loss!

I have a 6.5 foot high rubber tree indoors that I have had for over 25 years. It always dropped a leave or two maybe once a week. Most would be yellowing or all yellow and dry in appearance. The last month it has been losing about seven leaves per week and some are falling off as green fine looking ones. When they come off no sap is running so the seem to be cauterized at the point where they enter the trunk. I have looked and it appears there are no insects on it or in the soil. Help please the leave loss is not matching production and it is getting thin!
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