Any Plumbers in the house?

[FireDrgn]

Yes but (in a case of "not there before, but there now") usually from accidental damage that he would/should know about being done to that valve's stem - why Im asking if he can recall hitting/knocking/prying on it during the hot water repair (or, hopefuly not, his or someone elses knee or head during a fall, ouch).

Possible the seal around the stem on the cold valve is bad now or was going bad and the hot water repair something happened to of cr@ed it out.  If this is true, he likely should see water comming out of the valve at the stem now.

There is alot of things it could be.  I would start with the cold stem. stem includes the washer as copper pointed out.  Typically anything that should affect both sides would be check after a few other things are looked at.   "not there before , but there now."    Cold side stem is still the best place to start . as it only affects the cold side it will typically be within the cold side only.  Loose stem or debris will be close by.  Either way you have to pull the cold side stem.

[Shuffler]

Common problem, everyone's overthinking. pull your stems, make sure the washers are seated correctly and make sure the screws holding the washers are tight. Sounds like washer vibration to me.

Was my thought too

[bat21]

im not sure if you shut off main or had separate shutoff valves for the fixture, if separate valves were shutoff make sure they are both fully open, if partially open the washers sometimes vibrate, good luck

[mbailey]

I bet Plumbers love the weekend handyman. 

So do the insurance adjusters 

[FiLtH]

      Sounds like a loose seat. Also if you have a tempering valve for the toilet to prevent sweating, sometimes they can cause a racket if they screw up. This would be located near the toilet and have both hot and cold lines going to it, the mix side feeding to toilet.

[Drano]

Common problem, everyone's overthinking. pull your stems, make sure the washers are seated correctly and make sure the screws holding the washers are tight. Sounds like washer vibration to me.

^^this^^

AND....

Sounds to me like your problem isn't on your tub faucet's washers but with those in the shut off valves ahead of it. That is, of course, assuming you've checked your work on the tub faucets and you're sure that those washers are tight. A loose washer will tend to rattle around like that as pressures change in the system when you run other fixtures. With the valve open it's free to rattle around as much as it likes. The piping in your house will tend to amplify the sound and annoy the hell out of you. Start with any valves you might have operated in order to work on your tub. Hopefully it isn't the main shut off at the meter in which case you'll have to turn it off outside in the ground. Might have to get your water supplier to do this for you that way if that valve breaks in the gound (happens) it's on them.

And then he created the ball valve and he saw that it was good.

[mbailey]

With a name like Drano.......how can he be wrong?