basically my box is good,no crunches or whines,but it wouldnt go into 4th gear

i searched on here and vids on youtube etc,and came to conclusion id attempt it myself as it was leaking aswell and i bought new seals

so i removed box stripped rear end from it and removed top plate and inputshaft which has the 4th gear attached.before i done this i tried the selector hubs and could see why it wasent going into 4th,the 3 hub retainer/shift plates werent pushing down properly and were hitting synchro

so i have another box that wasent the best and i stripped that the same,took the retaining ring from 4th gear shift plates and the synchro was in better shape,

put these into my box and refitted inputshaft etc,sealed all parts to be refitted,i used the top cover with forks as my 1 had a hairline crack in it

fitted back to car filled with dex 3 etc,ran the car on axle stands and yes its fine now going into 4th gear and every other gear but something isnt right now

when i revved it in gear on stands in every gear it was fine in 1-3 but 4th and 5th have a horrible rattling like noise once i put load on box,and it never done this before.

now i could understand if i hadent done something right if it was 3rd and 4th as they share the same change hub but 5th???i never went near 5th gear as total seperate

it doesnt sound like a bearing noise to be fair,its like the actual cog/gear

1 thing i did notice was the inputshaft had zero play in it once i put it back together,where as it did have a little play before,surely the front seal doesnt cure that?

also would changing the top cover with shift forks make a diffrence?could i of not put this on correctly lined up inside? if it goes in every gear i doubt that

im no gearbox expert but i didint want to send it away and be done for £500 for 1 circlip as this has cured my original issue

any help appreciated