I have a two story house with a bathroom on the second floor, the issue I'm describing only occurs at the this bathroom no where else.
Once I turn on the hot water for the shower or sink faucet (both single lever) severe knocking occurs within 15 seconds of the water being turned on. This continues while the hot water is running and even after the water has been turned off for up to 3 minutes. The banging is steady... every few seconds.
The only way to stop it is to wait for that 3 minutes or FLUSH the toilet.
Any help would be appreciated.

Is this a hammer rattle or a knocking sound? This happens on both sinks that are located bin one cabinet, Correct? No hammer when you shut off a hot water side? Any action on the cold side? Back to you, Tom

More of a knocking sounds. It happens at both the sink faucet (single lever) and shower faucet (single lever)... hot water only. There is no hammer action when the valve is shut off just continued knocking for approx. 3 min when it eventually stops. I don't experience this on the cold side other than the knocking stops immediately after flushing the toilet in the same bathroom... but if I turn on the hot water it starts back up unless I flush again or wait it out.

Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Good luck and thank you for rating my answer. Tom

So you don't think it has anything to do with my gas hot water tank as I've read in some previous posts? Can you explain why flushing the toilet causing the banging to stop immediately?
I don't mind opening the walls/ceiling as you suggested, it's not my favourite thing to do but the banging gets so loud I'm afraid it might burst a joint one day... it's that loud at times.

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So you don't think it has anything to do with my gas hot water tank as I've read in some previous posts?

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The posts you read refer to knocking that comes from the gas hot water heater.
Did I misread your complaint? Is the water heater near your sinks?

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why flushing the toilet causing the banging to stop

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Opening ANY tap, including the toilet, causes a imbalance throughout the entire system. Is t5he knocking in the walls? the faucets or the water heater?? Back to you, Tom

The water heater in the basement almost directly above the 2nd floor washroom where the problem is occurring but the banging is occurring within the walls and is loudest on the main floor and in the 2nd floor bathroom.
Would you recommend soldering the copper clips to the pipes or just making sure they tight and enough of them?

Try this before you tear out walls or or use solder which is a lousy idea.

What Causes Hot-Water Faucet Hammering? | eHow

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Originally Posted by talaniman

Try this before you tear out walls or or use solder which is a lousy idea.

What Causes Hot-Water Faucet Hammering? | eHow

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I read the link you provided but this refers to water hammer and I don't believe that's my problem as the banging occurs while the hot water is running and shortly after its been turned off.
I have (on many occasions) drained the entire system cold and hot lines (not the hot water tank) but this doesn't help.
I don't believe my lines are fitted with cushions but they do have extensions of approx. 12-16" from what I recall.

The link also includes several solutions for different water hammer conditions and you do have one of them. The problem is you cannot find the particular pipe(s) that make the noise to anchor/insulate it if needed. The easiest way is to drain the heater to allow air into the tank to supply that cushion.

Can we assume this is a condition that just happened and has gotten worse? Read the other links on the side and let me know which best fits your condition and we can eliminate and narrow down the trouble shooting process. You may end up installing a pressure damper to solve your condition. It may be you have one and don't know it.

When you say you drained the lines but not the tank does that mean you just shut off the water and opened the faucets? Have you ever flushed your heater from the bottom spigot? Are any of those extensions capped? If they are those are your air cushions so please clarify.