To answer my question, I'm sure it is serious, but not sure at what level.

1990 4Runner, 3VZE, Auto

I drove this thing from Fresno to Salt Lake City last week and had only one minor problem that was easily fixed. For the last week or so this truck has been running beautifully. In fact, it still is and I'm very happy about that. However, last night it made a noise that is a lot like hitting the "voom voom" strips on the freeway, but a little higher pitch. Another way to describe it is the vibration of a computer fan. It only did it a few times for a few seconds each time. The first time I thought it was part of the song on the radio. The next few times the radio was not on.
This morning I jacked it up and checked the front wheels for any kind of movement, up and down, side to side, and I'm pretty sure the wheel bearings are in good shape. That leaves the axle shafts. I replaced these some years ago as an entire replacement, so the boots were already sealed and greased. The boots look good, no splits and no liberated grease. Nevertheless, I think that's where my problem is. Last night the temp was about 38F and was even colder the first time it made this racket.
Am I going to need to deal with this before I head back to Fresno? Will it stop doing it once I get to warmer weather? I'm not certain, but I think each time it happened the transmission has kicked out of OD (presuming cold weather), so I'm certain the cold is a factor, but how much do I need to worry?

A follow-up question is whether or not I can remove the end caps and fashion some replacement covers that won't engage the axles, or if I would cause more problems than I imagine solving?

I've also thought about putting some manual hubs on it so it will freewheel. Can I do that without any other mods?

ThanX in advance. I'm, planning to hit the highway on Monday morning unless I need to make repairs. I have access to a garage, which is probably the luckiest thing with this problem, just the timing is lousy.

 

from what I've heard CV's will make a clicking noise when they go bad, especially when you turn, and if you are in 4wd. Its hard to tell without hearing it, but your description sounds kinda like what my U-joint sounded like when it went bad. That was especially noisy when accelerating, then not too noticeable.

 

It turned out not to be a CV joint. Once I had a chance to get looking at it I found a dry U-joint with a missing grease fitting. With a new fitting and fresh grease, the noise magically stopped. It was nice to have an easy fix.

ThanX

 

Glad it was an easy fix! As for the manual hubs, it's an easy mod. Unbolt the flange plates and bolt on the hubs. You'll have to disassemble the hubs to do it and they'll need periodic maintenance. I did mine over 12 years ago and haven't ever considered going back. I did not disable the ADD, so running locked is like running it stock.