I've got a second hand bike. Doubt it ever had a chain replaced. Before I replaced the chain, it had an awful grinding sound. On replacement the machine worked perfectly and smoothly. (It's a Shimano altus 8 speed with a 3 chainring and an alvio rear derailleur) I thought it would be good to also replace the cassette. I bought the exact same model. Now it causes grinding sounds when not in the smallest cog. It always grinds in chainring 1 and 3. In chainring 2 it grinds in the larger cogs and had an awful vibration through the pedals in the smaller ones. My chainring looks okay I think ? What could be worth giving a go, I've tried to adjust the limit screws, and b is at its max. Any help would be great, just not sure what's going on and it worked great until I installed a new casette .
That really sounds like an indexing issue. The cable tension isn't as it should be. Barrel adjusters at the shifter or where the cable enters the rear derailleur can be turned to fine-tune how the chain runs on the cassette. Probably best to start over by releasing the inner cable from the pinch bolt, set the high limit screw--and there-by have the upper jockey wheel in the proper starting position. Reset the barrel adjuster(s) to one or two rotations out from full-in, and then route the loose inner cable properly through the pinch bolt area, taking up the slack in the cable and run in the pinch bolt firmly.
From there, attempt shifting up the cassette. Incomplete shifts call for increasing cable tension. Grinding in the gears after a shift means the cable tension isn't correct, but could also be too tight rather than loose. Adjust appropriately at one of the barrels. Generally the upper jockey wheel should run directly under the sprocket associated with a particular shift. Initial position of the jockey wheel has it running just under the outside plane of the smallest sprocket.
Obviously this all assumes correct installation of derailleur onto a straight hangar. The B-tab or B-screw needs to be touching the hangar's lip and eventually also set correctly when gearing is at its lowest. You'll need at least 5mm of clearance between the upper jockey wheel and large sprocket. Sometimes more, depending on size of largest sprocket.
An older bike with an Altus set-up may have non-indexed front shifters. Despite many of these models having click-detents, they are really just friction shifters. If the chain noise is heard it can mean the position of the front derailleur cage needs to be adjusted by manipulating the shifter. Worry about proper shifting/indexing at the cassette first. But if the chain noise is coming from chain rubbing on front derailleur cage, manipulate it out of the way via the left, front shifter.
Chain noise refractory to indexing can infer a bent hangar, derailleur cage, improper cassette install. The inner cable, ferrules, external housing are not infrequent areas where shift problems arise. This still sounds mostly indexing associated. Certainly should be tried very near first thing.
I'm assuming too that you didn't run the new chain on the old cassette for a long length of either time nor miles which can wear the chain abnormally making its interface with a new cassette compromised.
The old chain caused wear on the old cassette and chain wheels. So the new chain doesn’t exactly fit into the worn gears. Change the chainrings and the cog set. If this is too expensive, and it would be for me, ride as it is and the wear in will take care of itself. But I think you can see if it is properly adjusted as above, you still have an incompatibility with new and old. And a wear issue. HVAC