water pipe in bathtub makes very loud noise but when switched to the shower it stops

The diverter valve (which switches between the spout and the shower head) has rubber seals in it which can wear out or the thread holding it may not be tight and that makes the moving part of the valve vibrate. Can't really be more specific because the diverter can be either in the spout or it can be a separate device with it's own handle, or the feature could be a part of the valve cartridge - need to know the brand/model of the faucet.  Check if there's any loose thread and tighten if there is. If not, see if you can remove the diverter to take a closer look at its seals. Post the brand/model here, I'll see if I have the manual for that faucet.

If all else fails, try to lower the water pressure on the house main water line pressure reducing valve and see if it helps reducing the noise (balancing that out with having enough pressure to comfortably use the shower in the first place)