Part 1
Purchased a house and HW unit is in closet near hall bath.
When the cold water is shut off from lav, tub, or toilet, I hear a slight knocking sound... seems to be coming from the HW unit.

It's not loud and I'm pretty sure it is not the type of water knock sound you here when you do not have those extension pieces on your water lines to prevent this (forget the correct name for that extension).

It kind of sounds like something is floating around and bumping every now and then.

Seems to only do it on cold side.

Part 2
The cold water line is pretty warm, is this normal ?

Thanks for any advice or info.

Would this be a gas WH? It's normal for the cold water supply to pick up some heat. It's called convection. However, It's not normal for all the cold water lines to feel warm.
So where do you feel yours? Back to you, Tom

I should have stated it is electric.

It is only the cold line above the HW unit that feels a little warm.

Any idea's about the knocking sound ?

Thanks

Gas or electric?

Thanks for the replys but I thought I answered that in my second post.

It is an electric hot water heater.

When the cold water is shut off from lav, tub, or toilet, I hear a slight knocking sound... s It could be that your air chambers,( shock arresters ) need recharging.
"Water hammer" happens when you abruptly shut off the water. You have at least 40 pounds of pressure pushing the water flow. The weight of the water hitting a 90 degree bend causes the sound that you hear. We put in air chambers to cushion the hammer much like the shocks on your car.
Back in the 60's we were required to to put air chambers on all bath lavatories and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will began to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them in a two bath home. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps you out and thank you for rating my reply. TOM

look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut.

I do not see what you mentioned under the handle.
The bottom part of the valve is flat, nothing below it.
The handle is a 1/4 turn to close water flow.

Are you referring to the nut that fits over the water feed hole, which is above the valve on my connection ?

I do not see what you mentioned under the handle.
The bottom part of the valve is flat, nothing below it.
The handle is a 1/4 turn to close water flow.

Are you referring to the nut that fits over the water feed hole, which is above the valve on my connection ?

This is the valve that I was referring to,(see image) and the ones you must take apart. You will find them under the cabinets of the bathroom vanity and the kitchen sink. Bleed and recharge the air chambers and I'll bet the knock goes bye-bye. Good luck, Tom

Bonnet nut = packing nut

Thanks for the picture and explanations.

The angle stops I have look fairly new. There is no bonnet or packing nut. Looks as a round, smooth surface behind the handle. Nothing to put a wrench on.

http://i819.photobucket.com/albums/zz111/lacogada/anglestop.jpg

There has to be a bonnet nut, that's how the manufacturer gets the stem in the valve. Admittedly yours looks like some kind of smooth edged fitting. All Tom wants you to do is let some air in the pipe so the water can drain out.

Just disconnect the supply tube and open the valve.

OK! To drain the air chamber turn the water off at the house stop and undo and pull apart the 1/4" raiser. Open the faucet above to reduce air lock band then drain the line. Do this with each angle stop under the vanity and kitchen sink. Good luck. Tom

Thanks for the replys but I still don't think the sound is coming from what you call "water hammer".

It seems as though the sound is coming from the HW unit.

Would "water hammer" be at the HW unit or near the angle stops ?

Does "water hammer" make 4 or 5 consecutive knocks with each knock diminishing in volume ?

I agree! It doesn't sound like water hammer which, as a rule, only bangs once, The next thing I would check is the cold water supply shut off valve. Shut the water off at the house stop and open up and remove the stem from the supply valve. Check the screw that secures the washer. If it
S loose it will give you "seat chatter". Let me know what you find. Good luck, Tom