85 300TD cold weather trans thump

[shear2001]

I sometimes experience a clunk when I slow down from my trans. This only seems to occur when the temp. gets below 40 deg F. It used to happen often until I found a leak in one of my diaphrams under the dash. Could this be a vacuum problem or do you think it may be something else. I was thinking of a faulty modulator valve? Thanks.... PS all my locks work and I don't have the problem when the wather warms up..