Engine Noise

At idle, in gear or neutral, my '91 c1500, 4.6L w/100k miles sounds like submarine in Hunt for Red Ocktober.

I wouldn't call it valve noise, although it may be. The sound repeats at a very short interval, say 1 sec or so. It appears as though it cycles with every engine turn. Saying the noise would be something like: wurr, 1 sec, wurr, 1 sec, wurr, etc. I'd noise is deep in tone.

This is my first Chevy, is this typical @ 100k or do I have a bigger problem?

Thanks.