If you're confident that the brakes are not causing the issue, the sound may be coming from something in the front wheel drive train. Does there seem to be any connection between the noise and 4WD operation.
What kind of hubs do you have? I think they're probably the automatic type. Make sure the hubs are freeing up as designed.
Check the u-joints associated with the front wheel drive. There are 4 total - one at the end of each axle and the two on the drive shaft.
I know you don't think it's the brakes, but make sure that the calipers are securely bolted to the caliper plates.
Do a good inspection of all front end suspension and steering components. Even though the noise is rhythmic - which would suggest a rotating assembly interference - that might be a symptom rather than the cause. Something in the steering or suspension may be loose/worn/shifting and impacting a rotating part.
Sorry, I know this hasn't been much help, but noises are extremely difficult to diagnose over the internet - especially ones of an intermittent nature. So, unfortunately, it looks like you might be spending some time on your back under the truck.
One suggestion that might actually help; you posted this in the sticky threads area of brakes section. That is kind of "off the beaten path".
You might get more response if you repost to the main body of "Brakes". Also, since you are unconvinced that it is even brake related, try posting down in one of the drivetrain related categories - like "Differential and Driveline".
Finally, here is an exploded view (and parts list) of the front wheel drive and front brake systems that should be applicable to your vehicle (1987 V1)
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